v.] IN THE THEORY OF NATURAL SELECTION. 279 



for the above-mentioned reasons, and also because the influences 

 are mostly changeable, and occur sometimes in one and some- 

 times in another direction. 



Hereditary individual differences must therefore be derived 

 from some other source. 



I believe that such a source is to be looked for in the form of 

 reproduction by which the great majority of existing organisms 

 are propagated : viz. in sexual, or, as Hackel calls it, amphigonic 

 reproduction. 



It is well known that this process consists in the coalescence 

 of two distinct germ-cells, or perhaps only of their nuclei. 

 These germ-cells contain the germ-substance, the germ-plasm, 

 and this again, owing to its specific molecular structure, is the 

 bearer of the hereditary tendencies of the organism from which 

 the germ-cell has been derived. Thus in amphigonic repro- 

 duction two groups of hereditary tendencies are as it were 

 combined. I regard this combination as the cause of hereditary 

 individual characters, and I believe that the production of such 

 characters is the true significance of amphigonic reproduction. 

 The object of this process is to create those individual differences 

 which form the material out of which natural selection produces 

 new species. 



At first sight this conclusion appears to be very startling and 

 almost incredible, because we are on the contrary inclined to 

 believe that the continued combination of existing differences, 

 which is implied by the very existence of amphigonic repro- 

 duction, cannot lead to their intensification, but rather to their 

 diminution and gradual obliteration. Indeed the opinion has 

 already been expressed that deviations from the specific type 

 are rapidly destroyed by the operation of sexual reproduction. 

 Such an opinion may be true with regard to specific characters, 

 because the deviations from a specific type occur in such rare 

 cases that they cannot hold their ground against the large 

 number of normal individuals. But the case is different with 

 those minute differences which are characteristic of individuals, 

 because every individual possesses them, although of a different 

 kind and degree. The extinction of such differences could only 

 take place if a few individuals constituted a whole species ; 

 but the number of individuals which together represent a 

 species is not only very large but generally incalculable. Cross- 



