188 NATURAL SCIENCE [September 
lated with reproductive power. After the splitting up of this parent 
form into a large and a small species, in each of which the colour 
marking was invariable, the variations in fertility in the larger form, 
as correlated with colour marking, may have ceased, owing perhaps 
to the conditions of environment having changed from a variable to 
a more constant state, and the species would now become constant 
in this respect. The smaller form, on the other hand, may still be 
in the course of splitting up into two or more other species, differing 
in respect of colour marking, and maybe, of other characteristics. 
Another not fully explained question with regard to the origin 
of species is that of the divergence of character. Why is it that 
in the course of evolution, species have widened out into diverse 
branches, and have not continued in merely linear series? This 
question of divergence has been examined somewhat fully by 
Gulick.1 Darwin seeks to answer the question “from the simple 
circumstance that the more diversified the descendants from any 
one species become in structure, constitution, and habits, by so 
much will they be better enabled to seize on many and widely 
diversified places in the economy of nature, and so be enabled to 
increase in numbers.”2 As Romanes points out,? this argument is, 
however, assailable in one particular, 7.e., it ignores the fact of the 
swamping effects of intercrossing. Thus, in Darwin’s own words, 
it is where specific forms “jostle each other most closely” in an 
overstocked area that Natural Selection will be enabled to act most 
favourably on any members which may depart from the common 
type. Now, any varieties formed under these conditions by the 
splitting up of a species will be almost inevitably swamped by 
their mutual intercrossing, unless there be some degree of sterility 
between them. Under these conditions, therefore, reproductive 
divergence can act at a great advantage, as not only can it 
originate varieties, but by the mere fact of so doing it ensures 
these varieties not being eliminated by the swamping effects of 
their mutual intercrossing. 
It is unnecessary on this occasion to show how the theory of 
reproductive divergence may be applied to the other questions and 
difficulties connected with the theory of Natural Selection as an 
explanation of the mechanism of the origin of species. Suffice it 
to say that to some points in connection with Geographical Distri- 
bution, with the origin of rudimentary organs and other questions, 
it offers most material aid. The objections to the theory itself, as 
far as they present themselves to me, seem to be but few, and of 
but little weight. One of the most obvious is the frequently made 
statement, that crosses between varieties generally produce indi- 
1 Journ. Linn. Soc. (Zool.), vol. xx., p. 189. 
2 «The Origin of Species,” p. 87. 3 Loc. cit., p. 385. 
