286 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



ferences in size, 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 micro- 

 scopes 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 

 external influences upon the organism. Hereditary variability 

 cannot however arise in this way at every stage of organic de- 

 velopment, 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 de- 

 veloped parts will in the same manner tend to become still 

 weaker. Amphigonic reproduction therefore ensures that 

 every character w^hich is subject to individual fluctuation must 

 appear in many individuals w^th 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 in- 

 crease 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 



