VARIATION 413 



been accepted by Brooks ; and the assumption of a diversity of 

 the hereditary substances contained in the male and female 

 germ-cells, on which his theory is based, is rendered untenable 

 if it is recognised that the idants of both cells remain precisely 

 the same as regards number and quality during the process of 

 amphimixis. It will not, therefore, be rash to conclude that the 

 few observations which seem to be in favour of the dissimilar 

 effect of male and female germ-cells are not convincing, 

 although we may not be able at present to explain them fully. 

 To these belong certain rare cases, which have, perhaps, not 

 been very accurately observed, and possibly form exceptions to 

 the rule that hybrids of two species are identical, whether the 

 father or mother belonged to the species A or B. 



If we are forced to reject the assumption of the dissimilar 

 action of the two germ-cells, together with that of an internal 

 force of transformation, we can only refer hereditary individual 

 variability to the inequality of external influences ; and it then 

 remains to be seen how such influences can produce hereditary 

 differences if sojnatogenic modifications are not hereditary : for 

 external influences act directly, and often exclusively, upon the 

 body^ and not on the germ-cells. 



In a former essay I have already attempted to show that the 

 constant occurrence of individual variability and the continual 

 transformation in the intermixture required by selection is 

 brought about by amphimixis, although it is not the primary 

 cause of this variability; and that the accomplishment of 

 sexual reproduction is even based in almost all the known 

 organic forms on this necessity of preserving and continually 

 remodelling the hereditary variability of individuals. I am 

 convinced that the two forms of amphimixis — namely, the 

 conjugation of unicellitlar, and the sexual reproduction of 

 multicellular organisms — are means of producing variation. 

 The process furnishes an inexhaustible supply of fresh com- 

 binations of individual variations which are indispensable to the 

 process of selection. 



Hatschek* has contested this view of the significance of sexual 

 reproduction, and states that • transformations of species occur 

 far too rarely to admit of their explaining such an uninterruptedly 

 active process as sexual reproduction.'' It seems to me, how- 



* B. Hatschek, ' Lehrbuch der Zoologie,' Jena, 1888, p. 10. 



