162 OUTLINES OF EVOLUTIONARY BIOLOGY 



its life as a single cell the fertilized ovum or zygote which is 

 strictly comparable to a unicellular protistan. Like the Amoeba 

 it divides (under favourable circumstances) repeatedly, but the 

 products of division, instead of separating from one another and 

 going each its own way as an independent, unicellular organism, 

 all remain together and co-operate with one another to form a 

 multicellular body of greater or less" complexity . The cells of 

 which this body is composed become differentiated and specialized 

 in various directions. In so doing the vast majority of them 

 lose their faculty for independent existence, and when their powers 

 of division have become exhausted the tissues into which they 

 are combined become worn' out and ultimately die. Thus the 

 body or soma, as a whole, must suffer death sooner or later. The 

 only cells which are, even potentially, exempt from this fate are 

 the germ cells. These, instead of becoming highly specialized 

 constituents of the soma, remain in the condition of more or less 

 independent Protista, and have the power of separating sooner 

 or later from the parent body. > Like most of the Protista 

 they have also the habit of conjugating in pairs and thereby 

 renewing their powers of cell-division, and- thus arise the zygotes 

 or fertilized eggs from which the new v individuals take their 

 origin. 



The eggs from which the individuals of different kinds of 

 plants and animals arise are for the most part extraordinarily 

 similar to one another. The ovum of a rabbit, as we have 

 already seen, is a minute nucleated mass of protoplasm about 

 ^Jo^h inch in diameter, that of a human being is a similar but 

 somewhat larger cell, and if the ova of a hundred different kinds 

 of mammals were mixed together it would be an extremely 

 difficult, if not impossible task to sort them all out, even after 

 the most minute microscopical examination. Even where con^. 

 spicuous differences exist, as between the eggs of a mammal and 

 those of a bird, these are due almost entirely to the development 

 of accessory features, such as protective envelopes and food-yolk, 

 which have little to do with the nucleated mass of protoplasm 

 which constitutes the really vital part of the egg. Yet each kind 

 of fertilized egg, if it develop, will give rise to an organism 

 resembling the parent from which it was itself derived. More- 

 over, the resemblance will not be merely a general one, it will be 

 specific, and probably even more than specific, for it may include 

 minute individual characters peculiar to one or other of the 



