Chapter XVII: The "philosophie Zoologique"

Lamarck's mature views on the theory of descent comprise a portion of his celebrated _Philosophie zoologique_. We will let him tell the story of creation by natural causes so far as possible in his own words.

In the _avertissement_, or preface, he says that his experience has led him to realize that a body of precepts and of principles relating to the study of animals and even applicable to other parts of the natural sciences would now be useful, our knowledge of zoölogical facts having, for about thirty years, made considerable progress.

After referring to the differences in structure and faculties characterizing animals of different groups, he proceeds to outline his theory, and begins by asking:

"How, indeed, can I consider the singular modification in the
structure of animals, as we glance over the series from the most
perfect to the least perfect, without asking how we can account for
a fact so positive and so remarkable--a fact attested to me by so
many proofs? Should I not think that nature has successively
produced the different living beings by proceeding from the most
simple to the most compound; because in ascending the animal scale
from the most imperfect up to the most perfect, the organization
perfects itself and becomes gradually complicated in a most
remarkable way?"

This leads him to consider what is life, and he remarks (p. xv.) that it does not exist without external stimuli. The conditions necessary for the existence of life are found completely developed in the simplest organization. We are then led to inquire how this organization, by reason of certain changes, can give rise to other organisms less simple, and finally originate creatures becoming gradually more complicated, as we see in ascending the animal scale. Then employing the two following considerations, he believes he perceives the solution of the problem which has occupied his thoughts.

He then cites as factors (1) use and disuse; (2) the movement of internal fluids by which passages are opened through the cellular tissue in which they move, and finally create different organs. Hence the _movement of fluids in the interior of animals_, and the _influence of new circumstances_ as animals gradually expose themselves to them in spreading into every inhabitable place, are the two general causes which have produced the different animals in the condition we now see them. Meanwhile he perceived the importance of the preservation by heredity, though he nowhere uses that word, in the new individuals reproduced of everything which the results of the life and influencing circumstances had caused to be acquired in the organization of those which have transmitted existence to them.

In the _Discours préliminaire_, referring to the _progression_ in organization of animals from the simplest to man, as also to the successive acquisition of different special organs, and consequently of as many faculties as new organs obtained, he remarks:

"Then we can perceive how needs (_besoins_), at the outset reduced
to nullity, and of which the number gradually increases, have
produced the inclination (_penchant_) to actions fitted to satisfy
it; how the actions, becoming habitual and energetic, have caused
the development of the organs which execute them; how the force
which excites the organic movements may, in the simplest animals, be
outside of them and yet animate them; how, then, this force has been
transported and fixed in the animal itself; finally, how it then has
become the source of sensibility, and in the end that of acts of
intelligence.

"I shall add that if this method had been followed, then _sensation_
would not have been regarded as the general and immediate cause of
organic movements, and it would not have been said that life is a
series of movements which are executed in virtue of sensations
received by different organs; or, in other words, that all the vital
movements are the product of impressions received by the sensitive
parts.[179]

"This cause seems, up to a certain point, established as regards the
most perfect animals; but had it been so relatively to all living
beings, they should all be endowed with the power of sensation. But
it cannot be proved that this is the case with plants, and it cannot
likewise be proved that it is so with all the animals known.

"But nature in creating her organisms has not begun by suddenly
establishing a faculty so eminent as that of sensation: she has had
the means of producing this faculty in the imperfect animals of the
first classes of the animal kingdom," referring to the Protozoa. But
she has accomplished this gradually and successively. "Nature has
progressively created the different special organs, also the
faculties which animals enjoy."

He remarks that though it is indispensable to classify living forms, yet that our classifications are all artificial; that species, genera, families, orders, and classes do not exist in nature--only the individuals really exist. In the third chapter he gives the old definition of species, that they are fixed and immutable, and then speaks of the animal series, saying:

"I do not mean by this to say that the existing animals form a very
simple series, and especially evenly graduated; but I claim that
they form a branched series,[180] irregularly graduated, and which
has no discontinuity in its parts, or which, at least, has not
always had, if it is true that, owing to the extinction of some
species, there are some breaks. It follows that the _species_ which
terminates each branch of the general series is connected at least
on one side with other _species_ which intergrade with it" (p. 59).

He then points out the difficulty of determining what are species in certain large genera, such as Papilio, Ichneumon, etc. How new species arise is shown by observation.

"A number of facts teaches us that in proportion as the individuals
of one of our species are subjected to changes in situation,
climate, mode of life or habits, they thereby receive influences
which gradually change the consistence and the proportions of their
parts, their form, their faculties, even their structure; so that it
follows that all of them after a time participate in the changes to
which they have been subjected.

"In the same climate very different situations and exposures cause
simple variations in the individuals occurring there; but, after the
lapse of time, the continual differences of situation of the
individuals of which I speak, which live and successively reproduce
under the same circumstances, produce differences in them which
become, in some degree, essential to their existence, so that at the
end of many successive generations these individuals, which
originally belonged to another species, became finally transformed
into a new species distinct from the other.

"For example, should the seeds of a grass or of any other plant
natural to a moist field be carried by any means at first to the
slope of a neighboring hill, where the soil, although more elevated,
will yet be sufficiently moist to allow the plant to live there, and
if it results, after having lived there and having passed through
several generations, that it gradually reaches the dry and almost
arid soil of a mountain side; if the plant succeeds in living there,
and perpetuates itself there during a series of generations, it will
then be so changed that any botanists who should find it there would
make a distinct species of it.

"The same thing happens in the case of animals which circumstances
have forced to change in climate, mode of life, and habits; but in
their case the influences of the causes which I have just cited need
still more time than the plants to bring about notable changes in
the individuals.

"The idea of embracing, under the name of _species_, a collection of
like individuals which are perpetuated by generation, and which have
remained the same as long as nature has endured, implies the
necessity that the individuals of one and the same species should
not cross with individuals of a different species.

"Unfortunately observation has proved, and still proves every day,
that this consideration is unfounded; for hybrids, very common among
plants, and the pairings which we often observe between the
individuals of very different _species_ of animals, have led us to
see that the limits between these supposed constant species are not
so fixed as has been imagined.

"In truth, nothing often results from these singular unions,
especially if they are very ill-assorted, and then the individuals
which do result from them are usually infertile; but also, when the
disparities are less great, we know that the default in question
does not occur.

"But this cause only suffices to create, step by step, varieties
which finally become races, and which, with time, constitute what we
call _species_.

"To decide whether the idea which is formed of the _species_ has any
real foundation, let us return to the considerations which I have
already explained; they lead us to see:

"1. That all the organized bodies of our globe are true productions
of Nature, which she has successively formed after the lapse of much
time;

"2. That, in her course. Nature has begun, and begins over again
every day, to form the simplest organisms, and that she directly
creates only those, namely, which are the first germs (_ébauches_)
of organization, which are designated by the expression of
_spontaneous generations_;

"3. That the first germs of the animal and plant having been formed
in appropriate places and circumstances, the faculties of a
beginning life and of an organic movement established, have
necessarily gradually developed the organs, and that with time they
have diversified them, as also the parts;

"4. That the power of growth in each part of the organized body
being inherent in the first created forms of life, it has given rise
to different modes of multiplication and of regeneration of
individuals; and that consequently the progress acquired in the
composition of the organization and in the shape and diversity of
the parts has been preserved;

"5. That with the aid of sufficient time, of circumstances which
have been necessarily favorable, of changes of condition that every
part of the earth's surface has successively undergone--in a word,
by the power which new situations and new habits have of modifying
the organs of living beings, all those which now exist have been
gradually formed such as we now see them;

"6. Finally, that, according to a similar order of things, living
beings having undergone each of the more or less great changes in
the condition of their structure and parts, that which we call a
_species_ among them has been gradually and successively so formed,
having only a relative constancy in its condition, and not being as
old as Nature herself.

"But, it will be said, when it is supposed that by the aid of much
time and of an infinite variation in circumstances, Nature has
gradually formed the different animals known to us, shall we not be
stopped in this supposition by the simple consideration of the
admirable diversity which we observe in the _instincts_ of different
animals, and by that of the marvels of every kind presented by their
different kinds of _industry_?

"Shall we dare to extend the spirit of system so far as to say that
it is Nature who has herself alone created this astonishing
diversity of means, of contrivances, of skill, of precautions, of
patience, of which the _industry_ of animals offers us so many
examples? What we observe in this respect in the simple class of
_insects_, is it not a thousand times more than sufficient to make
us realize that the limit to the power of Nature in nowise permits
her to herself produce so many marvels, but to force the most
obstinate philosopher to recognize that here the will of the Supreme
Author of all things has been necessary, and has alone sufficed to
create so many admirable things?

"Without doubt, one would be rash or, rather, wholly insensate, to
pretend to assign limits to the power of the first Author of all
things; but, aside from that, no one could dare to say that this
infinite power could not will that which Nature even shows us it has
willed"[181] (p. 67).

Referring to the alleged proof of the fixity of species brought forward by Cuvier in the _Annales du Muséum d'Histoire naturelle_ (i., pp. 235 and 236) that the mummied birds, crocodiles, and other animals of Egypt present no differences from those now living, Lamarck says:

"It would assuredly be very singular if it were otherwise, because
the position of Egypt and its climate are still almost exactly what
they were at that epoch. Moreover, the birds which live there still
exist under the same circumstances as they were then, not having
been obliged to change their habits.

"Moreover, who does not perceive that birds, which can so easily
change their situation and seek places which suit them are less
subject than many other animals to the variations of local
circumstances, and hence less restricted in their habits."

He adds the fact that the animals in question have inhabited Egypt for two or three thousand years, and not necessarily from all time, and that this is not time enough for marked changes. He then gives the following definition of species, which is the best ever offered: "Species, then, have only a relative stability, and are invariable only temporarily."

"Yet, to facilitate the study and knowledge of so many different
organisms it is useful to give the name of _species_ to every
similar collection of similar individuals which are perpetuated by
heredity (_génération_) in the same condition, so long as the
circumstances of their situation do not change enough to render
variable their habits, character, and form."

He then discusses fossil species in the way already described in