272 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



remained precisely the same, this substance cannot be readily 

 modifiable, and there is very little chance of the smallest changes 

 being- produced in its molecular structure, by the operation of those 

 minute transient variations in nutrition to which the germ-cells, 

 together with every other part of the organism, are exposed. The 

 rate of growth of the germ-plasm will certainly vary, but its struc- 

 ture is unlikely to be affected for the above-mentioned reasons, and 

 also because the influences are mostly changeable, and occur some- 

 times in one and sometimes 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 w 7 hich 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 reproduction 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 sig- 

 nificance 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 reproduction, cannot 

 lead to their intensification, but rather to their diminution and 

 gradual obliteration. Indeed the opinion has already been ex- 

 pressed 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 



