The Individual in the Animal Kingdom by Julian S. Huxley

Author:Julian S. Huxley [Huxley, Julian]
Language: eng
Format: epub
Tags: Major transitions in evolution; individuality; multicellularity; superorganismality; Julian Huxley; history of biology
ISBN: 9780262045377
Publisher: MIT Press
Published: 2022-04-05T00:00:00+00:00

III

Some Other Definitions of Animal Individuality

From time to time various definitions of individuality have been given by zoologists. Most of them are framed with little reference to the philosophical idea of individuality, and the result has often been that the term individual as defined by them, though applicable to some reality of zoology, can no longer be used without absurdities in its more popular but more correct and more original sense.

One of the most widespread definitions considers the individual as “the total product of a single impregnated ovum” (8 a, p. 59), that is to say as the sum of the forms which appear between one sexual act and the next. This would make all the polyps in a colony of hydroids, all the separate polyps budded off by a fresh-water hydra, all the summer generations of the aphis, together constitute but a single individual. Of recent years it has not found so much favour, but Calkins (2) has urged that it should apply to protozoa, declaring that all the separate cells arising by continued division from a single parent between one sexual act (conjugation) and the next, should be considered as one individual, no less than the cells of a metazoan like man, which too arise by continued division from a single parent, the ovum, and remain connected to form his body.

Of the various facts which make the hypothesis untenable, the chief are concerned with the artificial or accidental production of two or more co-existent organisms from a single ovum.

In most animals each single fertilized egg gives rise to a single embryo and this to a single adult organism: but in some, where this is the normal rule, more than one embryo may be accidentally or artificially formed from one egg, and in others this multiplicity is the usual course of events, even though most of their relations may grow up in the ordinary humdrum way—“one egg, one adult.”

Aberrations may occur even in man: there can be very little doubt that identical twins1 (to leave all double monsters out of account) arise from the two cells produced by the first division of a single fertilized ovum, which have accidentally been torn apart instead of staying united.

A very interesting variation on this is seen in the nine-banded armadillo (Dasypus novem-cinctus) which regularly produces “identical quadruplets” (14). Most mammals give birth to several young at one time, but usually each grows up from a separate and separately fertilized ovum and each is enclosed in its own set of embryonic membranes. The armadillo’s brood, however, like the identical twins in man, has only a single chorionic membrane, and the four resemble each other minutely. Always of the same sex, their measurements are identical; even the number of plates in their armour is constant to less than 1 per cent., though the range of variation from brood to brood may be 5 per cent. and more.2

Then comes Experiment and confirms our conclusions of observation. The egg when it develops outside the body of its parent (the rule with most of the lower animals) is at the mercy of the experimenter.

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