Gulick — Cross-infertility. 437 



the Glacial period instead of the Jura-Trias, the query comes 

 up for explanation — Why the eroding river did not cut through 

 the clays, and more deeply trench the sea-border region. It is 

 remarkable that there are no channels or trenches over this 

 border for the Delaware, Chesapeake and other rivers to the 

 south. This appears at first to be proof of no elevation in 

 the Glacial period. It may be good proof of this as regards the 

 southern part of the Atlantic border ; but it is incomplete for the 

 more northern, inasmuch as the Glacial deposits of the closing 

 Glacial and the Champlain periods would probably have oblit- 

 erated through the agency of ice and rivers any trenches that 

 had been previously made. 



Art. LY. — The Preservation and Accumulation of Cross- 

 infertility ; by John T. Gulick. 



In his work on " Darwinism " in a section entitled " The 

 Influence of Natural Selection upon Sterility and Fertility," 

 Mr. Wallace reaches the conclusion that " If it [the cross- 

 infertility] was so closely correlated with physical variations or 

 diverse modes of life as to affect, even in a small degree, a con- 

 siderable proportion of the individuals of the two forms in 

 definite areas, it would be preserved by natural selection." 

 (p. 178). That the infertility of an incipient species with its 

 nearest allies is often preserved and accumulated, no one can 

 doubt ; but there are, it seems to me, very strong reasons for 

 believing that this can never be due to natural selection. 

 Natural selection is the exclusive breeding of those best adap- 

 ted to the environment of the species, through the failure to 

 propagate of those that are less adapted ; and the separate breed- 

 ing of those that are equally adapted introduces a wholly 

 different principle. In order to produce the cumulative modi- 

 fication of a variety, selection, whether natural or artificial, 

 must preserve certain forms of an intergenerating stock to 

 the exclusion of other forms of the same stock I may select 

 bantams as the object of my attention for a few years, and 

 then excluding them, raise only Shanghai fowls ; but this is 

 not the form of selection by which these divergent races were 

 produced. Again, if rats should supplant mice in any country, 

 some persons might call it natural selection, but such natural 

 selection would modify neither rats nor mice. On the other 

 hand if certain variations of mice are better able than the rest 

 to escape their pursuers, they will leave the most numerous 

 offspring, and modification of species will commence. Now if 

 we turn to page 175 of Mr. Wallace's book, we find, that in 



