1397] REPRODUCTIVE DIVERGENCE 185 



from the marriage statistics of 81,000 couples in Buda-Pesth, it is 

 possible to obtain evidence supporting our theory. Thus from these 

 figures one may see that parents of similar ages are more fertile inter se 

 than parents of dissimilar ages. With very young mothers the most 

 fertile fathers are, on an average, from three to six years in advance 

 as to age ; but with increasing years of the mothers the ages of the 

 fathers become less and less in excess, till at about thirty years of age 

 they coincide. At greater ages they gradually become slightly in defect. 

 Though this greater mutual fertility of individuals like in respect of 

 age can be of no influence in modifying the species, or splitting it 

 up into varieties, yet it gives us reasonable ground to suppose that 

 the fertility may also be found on examination to be greater with 

 individuals similar in respect of some other characteristic. In such 

 a case there would be a tendency for two or more varieties to be 

 formed, unless there were some other agency counteracting it. 



It will be seen that the evidence adduced in favour of a partial 

 sterility sometimes existing between varieties of a species is, in the 

 case of animals at least, very meagre. The reason of this is not far 

 to seek. Thus wild animals, when placed in confinement, will not, 

 in the majority of cases, breed at all. Domesticated animals, on the 

 other hand, do not afford evidence of much value, for the reason 

 given above. Also, it is generally held that domestication of itself 

 tends to increase fertility, and so would overcome any tendency to 

 sterility of varieties. 



In order to obtain evidence as to the existence of- a diminished 

 fertility between varieties, I have made a considerable number of 

 observations on the effects of crossing the various colour varieties 

 of the sea urchins, Sphaerechimbs gramdaris and Strongyloccntrotus 

 lividus, and have found that from a given number of ova the num- 

 ber of blastulae and the number of larvae subsequently produced 

 are appreciably smaller for crosses of dissimilar colour varieties than 

 for those of similar ones. Also, the larvae produced are, on an aver- 

 age, about 5 per cent, smaller. As, however, it will be necessary 

 to repeat these observations a large number of times before the 

 proof of such a partial sterility can be considered quite unexception- 

 able, and as moreover I hope to be able to make similar series of 

 observations among other classes of the Animal Kingdom, it would 

 be premature at this point to refer to these investigations at any 

 greater length. 



It should be borne in mind that the theory of Reproductive 

 Divergence does not require that there should be a partial sterility 

 between the varieties of species in all cases, or in even the majority 

 of cases. It merely premises that such sterility does exist in cer- 

 tain instances, and that in these the members of the species will 

 gradually become more and more divergent in respect of one or more 



