SEX AND HEREDITY 463 



quoted showing similar examples of differentiation of sex (Fig. 

 335). The question naturally arises why should such progressions exist 

 in a plurality of distinct phyletic lines ? That the differentiation of 

 sex has occurred more than once indicates a probability that some 

 real advantage has prompted it. 



The advantage appears to lie in the fact that the larger the amount 

 of nutriment embodied in the egg, the better nourished the offspring 

 will be in its first stages, and the better accordingly will be the chance 

 of its passing successfully through the dangerous risks of youth. 

 But the larger the egg the less mobile it will be. Even in the liquid 

 medium into which the eggs of Algae are often shed, a large body is 

 less easily moved than a small one. We naturally associate with this 

 the fact that the larger eggs have lost their motility. This is, how- 

 ever, immaterial so long as the spermatozoids remain small and 

 actively motile, provided that the egg can influence their movements, 

 and so act as a centre of attraction to them. It has beein seen that 

 the eggs are able to do this (p. 386). Such advantages as follow from 

 the stimulus of fusion, and the pooling of the hereditary factors of 

 the two sexual cells can still be secured by these means. Thus the 

 nett advantage lies with the plant which can, without sacrificing 

 the benefits that follow from syngamy, secure also for its offspring an 

 increased probability of successful germination. Conjugating organ- 

 isms, with their small equivalent gametes, may be regarded as a plant- 

 proletariat that produces numerous offspring with little physiological 

 capital ; so that each individual must depend chiefly upon its own 

 efforts. The organism that shows differentiation of its gametes 

 with an enlarged well-nourished egg is like a capitalist, whose pro- 

 geny starts life well furnished with' an inheritance. Other things 

 being equal, ultimate success will lie with the latter. Both King- 

 doms of Living Beings show how successful the results of sex- 

 differentiation have actually been : for all their higher terms have 

 differentiated gametes. 



A large naked egg, such as that of Fucus, may be a successful 

 enough means of propagation in water. But it could not develop 

 into an embryo exposed to the drying influence of the atmosphere. 

 A necessary condition of Life on the Land is thus the protection in 

 one way or another of the egg and the embryo. In the Evolution 

 of Land-living Plants and Animals this necessity has played a 

 leading part. The result is seen in the various forms of internal embry- 

 ology : that is, the envelopment of the egg and of the embryo within 

 the tissues of the parent. This brings also the collateral advantage 



