240 THE PHILOSOPHY OF BIOLOGY 



the composition of this fluid in the different parts of 

 the sac ; but these variations cannot be great ; the 

 fluid is not really a nutritive one ; and the process of 

 maturation is not hurried. We can hardly believe that 

 the differences in morphology are due to these minute 

 environmental differences. We may indeed say that 

 we do not really study the germ cells when we measure 

 the diameter of the egg or investigate any other measur- 

 able character, for the real germ-plasm is the chromatic 

 matter of the nucleus. But this obviously begs the 

 whole question : all the parts of the egg that are 

 accessible to observation do vary, and ought we to 

 conclude that the parts which are not accessible do not 

 vary ? They must vary : the germ-plasm of each egg 

 must be different from that of all the others, for the 

 organisms which develop from these germs show in- 

 heritable differences. Further, can we contend that 

 such minute environmental differences as we have 

 indicated affect the germ-plasm ? Is it so susceptible 

 to external changes ? A high degree of stability of 

 the germ-plasm is postulated in the mechanistic 

 hypothesis that we have considered, and indeed every- 

 thing indicates that the specific organisation is very 

 stable. Can it then be upset by such minute differences 

 in the somatic environment ? 



But the germ-plasm is not really simple, says 

 Weismann ; it is a complex mixture of ancestral 

 germ-plasms. The individual fish that we were con- 

 sidering arose from an aggregate of determinants, and 

 half of these determinants were received from the male 

 parent and half from the female one. But each of 

 these parents also arose from a similar aggregate of 

 determinants, which again were received from both 

 parents, and so on throughout the ancestry of the fish. 

 It is true that the germ-plasms contributed by the 



