Chapter I: HAVE endeavoured to prove that the germ-substance proper must be (3)

If we accept the theory of the continuity of the germ-plasm as in the main correct, it appears that higher animals and plants are constructed of two kinds of elements, the somatic cells and the germ-cells; both owe their being to the germ-plasm of the ovum, but the former do not contain it complete but only in individual determinants[24], and therefore can never give rise again to the rank of germ-cells; the others contain the latent germ-plasm intact, and can therefore produce not only cells like themselves for a certain time by division, but have also the power, when they are mature and the necessary conditions have been fulfilled, of bringing forth a new individual of the same species. The former have only a limited length of life, they die--they must necessarily die--when the life of the individual to which they belong is at an end; the latter are potentially immortal, like the unicellular organisms, that is, they can in favourable circumstances give rise to the germ-cells of a new individual, and so on for all time, as far as we can see. The germ-plasm of a species is thus never formed _de novo_, but it grows and increases ceaselessly; it is handed on from one generation to another like a long root creeping through the earth, from which at regular distances shoots grow up and become plants, the individuals of the successive generations. If these conditions be considered from the point of view of reproduction, the germ-cells appear the most important part of the individual, for they alone maintain the species, and the body sinks down almost to the level of a mere cradle for the germ-cells, a place in which they are formed, and under favourable conditions are nourished, multiply, and attain to maturity. But the matter can also be looked at in an opposite light, and then the endless root of the germ-plasm, with its germ-cells ever forming new individuals, may be regarded as the means by which alone nature was able to create multicellular organisms, individuals of higher and higher differentiation and capacity, able to adapt themselves to all possible conditions, and to make the fullest use of all the possibilities of life.

[24] Boveri has recently made an observation upon the thread-worm of the horse, which points to the correctness of the conception of the germ-plasm. The two first segmentation-cells both receive the four chromosomes of the species, but, in one of the two, a portion of the chromatin breaks off and degenerates, or dissolves, at least as far as can be seen. The other cell retains the whole mass of chromatin, and from this there arise later the primitive genital-cells. In the germ-track, therefore--so we must interpret it--the whole of the germ-plasm is retained, while a part of it is withdrawn from the soma. I have only partly described the process, and I do not wish to enter in detail on an interpretation of it, since it seems to me obscure and to require further observations before an interpretation can be attempted with any confidence.

MR. EDWARD ARNOLD'S

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Transcriber's Notes:

On page 104, the link for 'Elymnias lais' has been corrected
from PlII to PlIII.

Illustrations have been moved out of mid-paragraph.

Variations in spelling and hyphenation are retained.

Punctuation has been retained as published.

Bold type is shown as =strong=.

Italics are shown thus: _sloping_.

Small capitals have been capitalised.