IN THE THEORY OP NATURAL SELECTION. 279 



colour, form, and the number or arrangement of cilia. It must be 

 admitted that we have not hitherto paid sufficient attention to 

 this point, and moreover our best microscopes are only very rough 

 means of observation when we come to deal with such minute 

 organisms. Nevertheless we cannot doubt that the individuals 

 of the same species are not absolutely identical. 



We are thus driven to the conclusion that the ultimate origin of 

 hereditary individual differences lies in the direct action of ex- 

 ternal influences upon the organism. Hereditary variability can- 

 not however arise in this way at every stage of organic develop- 

 ment, as biologists have hitherto been inclined to believe. It can 

 only arise in the lowest unicellular organisms ; and when once 

 individual difference had been attained by these, it necessarily 

 passed over into the higher organisms when they first appeared. 

 Sexual reproduction coming into existence at the same time, the 

 hereditary differences were increased and multiplied, and arranged 

 in ever-changing combinations. 



Sexual reproduction can also increase the differences between 

 individuals, because constant cross-breeding must necessarily and 

 repeatedly lead to a combination of forces which tend in the 

 same direction, and which may determine the constitution of any 

 part of the body. If, for instance, the same part of the body is 

 strongly developed in both parents, the experience of breeders tells 

 us that the part in question is likely to be even more strongly 

 developed in the offspring ; and that weakly developed parts will 

 in the same manner tend to become still weaker. Amphigonic 

 reproduction therefore ensures that every character which is sub- 

 ject to individual fluctuation must appear in many individuals 

 with a strengthened degree of development, in many others with 

 a development which is less than normal, while in a still larger 

 number of individuals the average development will be reached. 

 Such differences afford the material by means of which natural 

 selection is able to increase or weaken each character according 

 to the needs of the species. By the removal of the less well- 

 adapted individuals, natural selection increases the chance of 

 beneficial cross-breeding in the subsequent generations. 



Every one must admit that, if a species came into existence 

 having only a small number of individual differences which 

 appeared in the different parts of different individuals, the number 



