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 Komanes points out, 3 this argument is, 

 however, assailable in one particular, i.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 anil 

 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 Jourv. Linn. Roc. (Zool.), vol. xx., p. 189. 



2 " The Origin of Species," p. 87. 3 Loc. cit., p. 385. 



