FOUNDATION OF A. THEORY OF HEREDITY. 229 



beneficial ; and further, that at least in this group parthenogenesis 

 became possible, and was adopted, in each species as soon as it 

 became useful. Such a result can be easily understood if it is only 

 the presence of more or less germ-plasm which decides whether an 

 egg- is, or is not, capable of development without fertilization. 



If we now examine the foundations of this hypothesis we shall 

 find that we may at once accept one of its assumptions, viz. that 

 fluctuations occur in the quantity of germ-plasm in the segmen- 

 tation nucleus; for there can never be absolute equality in any 

 single part of different individuals. As soon therefore as these 

 fluctuations become so great that parthenogenesis is produced, it may 

 become, by the operation of natural selection, the chief mode of 

 reproduction of the species or of certain generations of the species. 

 In order to place this theory upon a firm basis, we have simply to 

 decide whether the quantity of germ-plasm contained in the seg- 

 mentation nucleus is the factor which determines development ; 

 although for the present it will be sufficient if we can render this 

 view to some extent probable, and show that it is not in contra- 

 diction with established facts. 



At first sight this hypothesis seems to encounter serious diffi- 

 culties. It will be objected that neither the beginning nor the end 

 of embryonic development can possibly depend upon the quantity 

 of nucleoplasm in the segmentation nucleus, since the amount may 

 be continually increased by growth ; for it is well known that 

 during embryonic development the nuclear substance increases 

 with astonishing rapidity. By an approximate calculation I found 1 

 that, in the egg of a Cynips, the quantity of nuclear substance 

 present at the time when the blastoderm was about to be formed, 

 and when there were twenty-six nuclei, was even then seven times 

 as great as the quantity which had been contained in the seg- 

 mentation nucleus. How then can we imagine that embryonic 

 development would ever- be arrested from want of nuclear sub- 

 stance, and if such deficiency really acted as an arresting force, how 

 then could development begin at all ? We might suppose that 

 when germ-plasm is present in sufficient quantity to start segmen- 

 tation, it must also be sufficient to complete the development ; for 

 it grows continuously, and must presumably always possess a power 



1 Weismann, 'Beitrage zur Kenntniss der ersten Entwicklungsvorgange im 

 Insectenei,' Bonn, 1882, p. 106. 



