1897] 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, Sphaerechinus granularis and Strongylocentrotus 
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 
