Chapter III (2)

It is a familiar experience to the student, not only of the Mollusca, but of every branch of animal or vegetable life, to come across examples which exhibit certain slight deviations from the type form as usually understood. These deviations may be more or less pronounced, but, as a rule, a series of forms can be discovered, gradually leading up to or down from the type. The definition of what constitutes a species,--and, still more, the rigid application of such definition--will always remain a difficult task, so long as the personal element persists in him who defines.[183] What seems to one authority ample ground for distinction of species, another may regard as of comparatively trivial importance. The practical outcome of these divergent views is sufficiently illustrated by the attitude of Mr. F. P. Marrat on the one hand, and of what may be called the modern French school of conchologists on the other. Mr. Marrat holds, or held, that the great genus _Nassa_, of which more than 150 species are generally recognised, is one shell (species) in an endless variety of forms. The modern French school go to the other extreme, and apparently proceed upon the view that almost any difference in form, however slight, is sufficient to constitute a separate species.

It will be generally admitted, however, that some _structural difference_ in the organisation of the animal (as distinct from that of the shell alone) is necessary for the permanent constitution of specific rank.[184] What _amount_ of structural difference is required, what particular organ or organs must exhibit this difference, will depend largely upon the idiosyncrasy of the observer. But if this, or something like this definition of a species be accepted, it will follow that a so-called ‘variety’ will be a form which exhibits differences from the type which do not amount to permanent structural differences in the organisation of the animal. The final court of appeal as to what affords sufficient evidence for ‘permanent structural differences’ will have to be, as with Aristotle of old, the judgment of the educated man.

It is, however, more to our present purpose to discuss the _causes of variation_ than to lay down definitions of what variation is. One of the most obvious causes of variation lies in a change or changes in the environment. If we may assume, for the moment, that the type form of a species is the form which is the mean of all the extremes, and that this form is the resultant of all the varied forces brought to bear upon it, whether of food, climate, temperature, competition of numbers, soil, light, amount of water, etc., it will follow that any change in one or more of these forces, if continuous and considerable, any change, in other words, of the environment, will produce its effect upon the organism in question. And this effect will be for the better or for the worse, according to the particular nature of the change itself as tending towards, or away from, the _optimum_ of environment for the species concerned. Hence may be produced varieties, more or less marked according to the gravity of the change, although it must be noted that at times a change apparently unimportant from our point of view, will produce very marked results upon the species. It is indeed scarcely possible to predict with any certainty, in the present state of our knowledge (beyond certain broad results) what will be the particular effect upon a species of any given change in its surroundings.

=Effects of Change in the Environment as tending to produce Variation.=

(_a_) _Changes in Climate, Temperature, Elevation, etc._--In the eastern basin of the Baltic the marine Mollusca are much more stunted than in the western.[185] For instance, _Mytilus edulis_ near Kiel is 8–9 cm. long, while near Gothland it only attains a length of 3–4 cm. Mollusca living at only a shallow depth (_e.g._ _Tellina balthica_, _Mya arenaria_, _Cardium edule_) do not differ much in size in different parts of the Baltic, but in the far eastern basin the calcareous layers of the shells of _Mya arenaria_ and _Tellina balthica_ are extraordinarily thin, and disappear very rapidly after death, leaving only the cuticular membrane, still united by the ligament, in a perfect state of preservation. These remarkable variations are no doubt to a large extent due to the violent changes of temperature which are experienced in the Baltic, and by which the steady development of the animals in question is interrupted and thrown out of gear. The same species occur on the coasts of Greenland and Iceland, where they attain a considerably larger size than in the Baltic, in spite of the lower mean temperature, probably because their development is not interrupted by any sudden change from cold to heat or _vice versâ_.

Karl Semper has shown that _Limnaea stagnalis_ is developed, lives and feeds best in a mean temperature of about 20° C. (= 68° F.). This mean, however, must not be the mean of two distant extremes, for the _Limnaea_ cannot digest its food and grow in a temperature which is less than 14° or 15° C. (= 57° or 59° F.), or more than 30° to 32° C. (= 86° to 90° F.). In certain localities, therefore, the interruption to the growth of this species must be serious and prolonged, and may tend towards the production of more or less dwarfed varieties. Thus specimens from Malham Tarn, a lake in Yorkshire 1250 feet above the sea, are permanently dwarfed, and have a very thin and fragile shell. _Limnaea peregra_ in the Pyrenees, Alps, and Himalayas is generally of a very delicate form and dwarfed habit, while the small variety known as _lacustris_ occurs, according to Jeffreys, only in mountain lakes in Zetland, Scotland, Ireland, and N. England. Specimens brought by Mr. Bateson from lakes near the Sea of Aral, which are salt for some months and comparatively fresh for others, exhibit clearly the effect of changes in the environment (Figs. 33 and 34). Excess of heat produces similar results to excess of cold. _L. peregra_ var. _thermalis_, found in the warm springs of the Pyrenees and the Vosges, and the var. _geisericola_, from the hot water of the Iceland geysers, are alike thin and dwarfed forms.

Many instances may be given of ‘varieties due to locality.’ In some of these, the cause which predisposes towards variation can be inferred with some approach to certainty, in others we must be content to note the fact, without at present being able to perceive its explanation.

Desert specimens of widely distributed species, _e.g._ _Helix pomatia_, _H. niciensis_, _H. pisana_, _Leucochroa candidissima_ are much thicker than the type, and tend to lose all trace of coloured bands. These modifications are clearly the means of preventing evaporation of moisture, the dull white or grayish brown colour being calculated to absorb the smallest possible amount of heat. Desert shells in all parts of the world (_e.g._ N. Africa, Arabia, Central Asia, S. Africa, W. America) have been noticed to exhibit these peculiarities.

A very singular case of the reverse process, _i.e._ the production of darkened forms of shell through cold, has been noticed by Fischer as characteristic of the marine shells of the west coast of South America.[186] This melanism is especially noticeable in _Trochus_, _Turbo_, _Chiton_, _Mitra_, and _Pleurotoma_, and is attested by the specific names, not merely expressive of actual blackness (_e.g._ _nigerrimus_, _ater_, _atramentarius_, _maurus_), but also of a generally lugubrious tone (_e.g._ _moestus_, _funebralis_, _tristis_, _lugubris_, _luctuosus_). It is highly probable that this concurrence of specific melanism (which stands quite alone in the world) is due to the cold polar current which impinges on the Chilian coasts, for the same genera occur on the opposite shores of the continent without exhibiting any trace whatever of this mournful characteristic.

It is a well-known fact, attested by many observers, that our common _Limax agrestis_ as well as the young of _Arion ater_ become decidedly darker in summer than in winter. If these slugs were accustomed to disport themselves in the sun, it might have been suggested that this increased darkness of colour tended to absorb more of the heat rays. But since this is not the case, the result is probably due to some unexplained effect of higher temperature. According to Lessona and Pollonera, the length of the keel in _Limax arborum_ varies greatly in different parts of Italy, being shorter in specimens from low ground, but much longer in those inhabiting more elevated regions. The longer the keel, the more obscure the colouring becomes, so that in the Upper Alps of Piedmont individuals are practically black. Roebuck has observed that Scottish specimens of this same slug are much darker and less translucent than English forms. According to Simroth, our common black slug, _Arion ater_, is a northern type, which in more southern latitudes assumes the form known as _A. rufus_. Similarly _Limax maximus_ “in its northern form _cinereo-niger_ is almost wholly black, but in the more genial climate of Italy develops a series of brilliantly coloured and strikingly marked variations which have received numerous distinctive names from Italian limacologists.”[187] According to Scharff, however[188] (who regards the colours of slugs as in the main protective), these dark forms are by no means exclusively northern, being found equally on the parched plains of Spain and Portugal, and in the bleak climate of Norway. The same authority observes that similar forms occur both in the dry regions of E. Germany, and in the very humid district of western Ireland.

It appears unquestionable that marine genera from high northern latitudes are provided with shells of uniform colour, or whitish with a pale brown epidermis; spots, bands, or stripes seldom occur. The arctic forms of _Buccinum_, _Trophon_, _Chrysodomus_, _Margarita_, _Crenella_, _Leda_, _Yoldia_, _Astarte_ illustrate this fact. In the more temperate seas of Europe, colours tend on the whole to increase, although there are certain genera (_e.g._ _Pecten_) which are not more brightly coloured in Mediterranean than in Icelandic waters.

Land Mollusca inhabiting the mainland of a continent not unfrequently become smaller when they have spread to adjacent islands where perhaps the rainfall is less abundant or the soil and food-supply less nicely adjusted to their wants. _Orthalicus undatus_ is decidedly larger on the mainland of S. America than on the adjacent islands of Trinidad and Grenada. Specimens of _Bulimulus exilis_ from Barbados are invariably broader and more obese than those from S. Thomas, while those from the volcanic island of S. Lucia, where lime is deficient, are small and very slender. _Streptaxis deformis_, as occurring at Trinidad, is only half the size of specimens from Georgetown, Demerara.[189]

Certain localities appear, for some unexplained reason, to be particularly favourable to the production of albino varieties. The neighbourhood of Lewes, in Sussex, has produced no fewer than fourteen of these forms of land Mollusca and five of fresh-water.[190]

Our common _Helix aspersa_, as found near Bristol, is said to be ‘dark coloured’; about Western-super-mare ‘brown, with black markings’; near Bath ‘very pale and much mottled’; at Cheddar ‘very solid and large.’[191] Sometimes the same kind of variation is exhibited by different species in the same locality. Thus specimens of _H. aspersa_, _H. nemoralis_, and _H. hortensis_, taken from the same bank at Torquay, presented a straw-coloured tinge of ground colour, with red-brown bands or markings. Trochiform _H. nemoralis_ and _H. arbustorum_, sinistral _H. hortensis_ and _H. aspersa_, sinistral _H. aspersa_ and _H. virgata_, and similarly banded forms of _H. caperata_ and _H. virgata_, have been taken together.[192]

The immediate neighbourhood of the sea appears frequently to have the effect of dwarfing land Mollusca. Thus the var. _conoidea_ of _Helix aspersa_, which is small, conical, with a compressed mouth, occurs ‘on sandhills and cliffs at the seaside.’ The varieties _conica_ and _nana_ of _Helix hispida_ are found ‘near the sea.’ _Helix virgata_ is exceedingly small in similar localities, and tends to become unicoloured. _H. caperata_ var. _Gigaxii_, a small depressed form, occurs at ‘Sandwich and Falmouth.’[193] Sometimes, however, the exact opposite is the case, for _H. nemoralis_ var. _major_, which is ‘much larger’ than the type, occurs on ‘sandhills and downs’ and is ‘remarkably large in the I. of Arran, Co. Galway.’ The dwarf form of _Limnaea peregra_ known as _maritima_ appears to be confined to the neighbourhood of the sea.

Dwarfing of the shell seems frequently to be the result of an elevated locality, not perhaps so much as the direct consequence of purer air and less barometric pressure, as of changes in the character of the food supply and in the humidity of the air. Several species of _Helix_ have a variety _minor_ which is characteristic of an Alpine habitat. _Helix arbustorum_ var. _alpestris_, which is scarcely two-thirds the size of the type, occurs on the Swiss Alps in the region of perpetual snow. Sometimes a very slight elevation is sufficient to produce the dwarfed form. At Tenby the type form of _Helix pisana_ is scattered in countless numbers over the sandhills just above high-water mark. At the extreme western end of these sandhills rises abruptly to a height of over 100 feet the promontory known as Giltar Head, the vegetation of which is entirely distinct from that of the burrows below. There is a colony of _H. pisana_ at the end of Giltar, all of which are devoid of the characteristic markings of the typical form, and most are dwarfed and stunted in growth.

Occasionally the same variety will be found to be produced by surroundings of very different nature. Thus the var. _alpestris_, of _H. arbustorum_ mentioned above, besides being characteristic of high Alpine localities, also occurs abundantly in low marshes at Hoddesdon on the River Lea. _Helix pulchella_ var. _costata_, according to Jeffreys, is found in dry and sandy places, often under loose stones and bricks on walls, while other authorities have noticed it in wet and dry localities quite indifferently.

Sometimes the production of a variety may be traced to the intrusion of some other organism. According to Brot, nine-tenths of the _Limnaea peregra_ inhabiting a certain pond near Geneva, were, during one season, afflicted with a malformation of the base of the columella. This deformity coincided with the appearance, in the same waters, of extraordinary numbers of _Hydra viridis_. The next season, when the _Hydra_ disappeared, the next generation of _Limnaea_ was found to have resumed its normal form.

It has been noticed that a form of _Helix caperata_ with a flattened spire and wide umbilicus is restricted to tilled fields, especially the borders of clover fields, while a form with a more elevated spire and more compact whorls occurs exclusively in open downs and uncultivated places. The Rev. S. S. Pearce accounts[194] for this divergence by the explanation that the flatter spire enables the shell of the fields to creep about more easily under the leaves or matted weeds, seldom requiring to crawl up a stalk or stem, while on the short turf of the downs and pastures the smaller and more rounded shell enables the animal to manoeuvre in and out of the blades of grass, and even to crawl up them with considerable activity. The same writer endeavours to explain the causes which regulate the distribution of _H. caperata_ var. _ornata_. He found that this variety (dark bands on a white ground) occurred almost exclusively on downs which were fed upon by sheep, associated with the common or mottled form, while the latter form alone occurred in localities where sheep were not accustomed to feed. Assuming then, as is probably the case, that sheep, in the course of their close pasturing, devour many small snails, he believes that individuals of the more conspicuous form _ornata_ were more likely to be noticed, and therefore avoided, by the sheep, than the mottled form, which would more easily escape their observation. Hence the var. _ornata_ is due to the advantage which strikingly coloured individuals obtained owing to their conspicuous habit, as compared with the typical form, which would be less readily detected.

(_b_) _Changes in Soil, Station, Character of Water, etc._--A deficiency of lime in the composition of the soil of any particular locality produces very marked effects upon the shells of the Mollusca which inhabit it; they become small and very thin, occasionally almost transparent. The well-known var. _tenuis_ of _Helix aspersa_ occurs on downs in the Channel Islands where calcareous material is scarce. For similar reasons, _H. arbustorum_ develops a var. _fusca_, which is depressed, very thin, and transparent, at Scilly, and also at Lunna I., E. Zetland.

(1) Felixstowe, sheltered coast; (2), (3) Newquay, on veined
and coloured rock; (4) Herm, rather exposed; (5) Solent, very
sheltered; (6) Land’s End, exposed rocks, small food supply;
(7) Scilly, exposed rocks, fair food supply; (8) St. Leonards,
flat mussel beds at extreme low water; (9) Robin Hood’s Bay,
sheltered under boulders, good food supply; (10) Rhoscolyn, on
oyster bed, 4–7 fath. (Macandrew); (11) Guernsey, rather exposed
rocks; (12) Estuary of Conway, very sheltered, abundant food
supply; (13), (14) Robin Hood’s Bay, very exposed rocks, poor
food supply; (14) slightly monstrous; (15), (16), (17) Morthoe,
rather exposed rocks, but abundant food supply; (18) St. Bride’s
Bay; (19) L. Swilly, sheltered, but small food supply. All from
the author’s collection, except (10).]

The common dog-whelk (_Purpura lapillus_) of our own coasts is an exceedingly variable species, and in many cases the variations may be shown to bear a direct relation to the manner of life (Fig. 35). Forms occurring in very exposed situations, _e.g._ Land’s End, outer rocks of the Scilly Is., coasts of N. Devon and Yorkshire, are stunted, with a short spire and relatively large mouth, the latter being developed in order to increase the power of adherence to the rock and consequently of resistance to wave force. On the other hand, shells occurring in sheltered situations, estuaries, narrow straits, or even on open coasts where there is plenty of shelter from the waves, are comparatively of great size, with a well-developed, sometimes produced spire, and a mouth small in proportion to the area of shell surface. In the accompanying figure, the specimens from the Conway estuary and the Solent (12, 5) well illustrate this latter form of shell, while that from exposed rocks is illustrated by the specimens from Robin Hood’s Bay (13, 14). Had these specimens occurred alone, or had they been brought from some distant and unexplored region, they must inevitably have been described as two distinct species.

Mr. W. Bateson has made[195] some observations on the shells of _Cardium edule_ taken from a series of terraces on the border of certain salt lakes which once formed a portion of the Sea of Aral. As these lakes gradually became dry, the water they contained became salter, and thus the successive layers of dead shells deposited on their borders form an interesting record of the progressive variation of this species under conditions which, in one respect at least, can be clearly appreciated. At the same time the diminishing volume of water, and the increasing average temperature, would not be without their effect. It was found that the principal changes were as follows: the thickness, and consequently the weight, of the shells became diminished, the size of the beaks was reduced, the shell became highly coloured, and diminished considerably in size, and the breadth of the shells increased in proportion to their length (Fig. 36). Shells of the same species of _Cardium_, occurring in Lake Mareotis, were found to exhibit very similar variations as regards colour, size, shape, and thickness.

_Unio pictorum_ var. _compressa_ occurs near Norwich at two similar localities six or seven miles distant from one another, under circumstances which tend to show that similar conditions have produced similar results. The form occurs where the river, by bending sharply in horse-shoe shape, causes the current to rush across to the opposite side and form an eddy near the bank on the outside of the bend. Just at the edge of the sharp current next the eddy the shells are found, the peculiar form being probably due to the current continually washing away the soft particles of mud and compelling the shell to elongate itself in order to keep partly buried at the bottom.[196]

The rivers Ouse and Foss, which unite just below York, are rivers of strikingly different character, the Ouse being deep, rapid, with a bare, stony bottom, and little vegetable growth, and receiving a good deal of drainage, while the Foss is shallow, slow, muddy, full of weeds and with very little drainage. In the Foss, fine specimens of _Anodonta anatina_ occur, lustrous, with beautifully rayed shells. A few yards off, in the Ouse, the same species of _Anodonta_ is dull brown in colour, its interior clouded, the beaks and epidermis often deeply eroded. Precisely the same contrast is shown in specimens of _Unio tumidus_, taken from the same rivers, Ouse specimens being also slightly curved in form. Just above Yearsley Lock in the Foss, _Unio tumidus_ occurs, but always dwarfed and malformed, a result probably due to the effect of rapidly running water upon a species accustomed to live in still water.[197] Simroth records the occurrence of remarkably distorted varieties in two species of _Aetheria_ which lived in swift falls of the River Congo.[198]

A variety of _Limnaea peregra_ with a short spire and rather strong, stoutly built shell occurs in Lakes Windermere, Derwentwater, and Llyn-y-van-fach. It lives adhering to stones in places where there are very few weeds, its shape enabling it to withstand the surf of these large lakes, to which the ordinary form would probably succumb.[199]

Scalariform specimens of _Planorbis_ are said to occur most commonly in waters which are choked by vegetation, and it has been shown that this form of shell is able to make its way through masses of dense weed much more readily than specimens of normal shape.

Continental authorities have long considered _Limnaea peregra_ and _L. ovata_ as two distinct species. Hazay, however, has succeeded in rearing specimens of so-called _peregra_ from the ova of _ovata_, and so-called _ovata_ from the ova of _peregra_, simply by placing one species in running water, and the other in still water.

According to Mr. J. S. Gibbons[200] certain species of _Littorina_, in tropical and sub-tropical regions, are confined to water more or less brackish, being incapable of living in pure salt water. “I have met,” says Mr. Gibbons, “with three of these species, and in each case they have been distinguished from the truly marine species by the extreme (comparative) thinness of their shells, and by their colouring being richer and more varied; they are also usually more elaborately marked. They are to be met with under three different conditions--(1) in harbours and bays where the water is salt with but a slight admixture of fresh water; (2) in mangrove swamps where salt and fresh water mix in pretty equal volume; (3) on dry land, but near a marsh or the dry bed of one.

“_L. intermedia_ Reeve, a widely diffused E. African shell, attaches itself by a thin pellicle of dried mucus to grass growing by the margin of slightly brackish marshes near the coast, resembling in its mode of suspension the Old World _Cyclostoma_. I have found it in vast numbers in situations where, during the greater part of the year, it is exposed to the full glare of an almost vertical sun, its only source of moisture being a slight dew at night-time. The W. Indian _L. angulifera_ Lam., and a beautifully coloured E. African species (_? L. carinifera_), are found in mangrove swamps; they are, however, less independent of salt water than the last.”

Mr. Gibbons goes on to note that brackish water species (although not so solid as truly marine species) tend to become more solid as the water they inhabit becomes less salt. This is a curious fact, and the reverse of what one would expect. Specimens of _L. intermedia_ on stakes at the mouth of the Lorenço Marques River, Delagoa Bay, are much smaller, darker, and more fragile, than those living on grass a few hundred yards away. _L. angulifera_ is unusually solid and heavy at Puerto Plata (S. Domingo) among mangroves, where the water is in a great measure fresh; at Havana and at Colon, where it lives on stakes in water but slightly brackish, it is thinner and smaller and also darker coloured.

(_c_) _Changes in the Volume of Water._--It has long been known that the largest specimens, _e.g._ of _Limnaea stagnalis_ and _Anodonta anatina_, only occurred in pieces of water of considerable size. Recent observation, however, has shown conclusively that the volume of water in which certain species live has a very close relation to the actual size of their shells, besides producing other effects. _Lymnaea megasoma_, when kept in an aquarium of limited size, deposited eggs which hatched out; this process was continued in the same aquarium for four generations in all, the form of the shell of the last generation having become such that an experienced conchologist gave it as his opinion that the first and last terms of the series could have no possible specific relation to one another. The size of the shell became greatly diminished, and in particular the spire became very slender.[201]

The same species being again kept in an aquarium under similar conditions, it was found that the third generation had a shell only four-sevenths the length of their great grandparents. It was noticed also that the sexual capacities of the animals changed as well. The liver was greatly reduced, and the male organs were entirely lost.[202]

K. Semper conducted some well-known experiments bearing on this point. He separated[203] specimens of _Limnaea stagnalis_ from the same mass of eggs as soon as they were hatched, and placed them simultaneously in bodies of water varying in volume from 100 to 2000 cubic centimetres. All the other conditions of life, and especially the food supply, were kept at the known optimum. He found, in the result, that the size of the shell varied directly in proportion to the volume of the water in which it lived, and that this was the case, whether an individual specimen was kept alone in a given quantity of water, or shared it with several others. At the close of 65 days the specimens raised in 100 cubic cm. of water were only 6 mm. long, those in 250 cubic cm. were 9 mm. long, those in 600 cubic cm. were 12 mm. long, while those kept in 2000 cubic cm. attained a length of 18 mm. (Fig. 37).

An interesting effect of a sudden fall of temperature was noticed by Semper in connection with the above experiments. Vessels of unequal size, containing specimens of the _Limnaea_, happened to stand before a window at a time when the temperature suddenly fell to about 55° F. The sun, which shone through the window, warmed the water in the smaller vessels, but had no effect upon the temperature of the larger. The result was, that the _Limnaea_ in 2000 cubic cm., which ought to have been 10 mm. long when 25 days old, were scarcely longer, at the end of that period, than those which had lived in the smaller vessels, but whose water had been sufficiently warm.