118 A PRIMER OF BIOLOGY 



by a on the one hand and b on the other. Either a 

 or 6 indeed, may again develop characters which in 

 some respects give it an advantage over the more 

 constant descendants of A, whose territory it will 

 therefore invade, and hence instead of having 

 one type in a particular locality we may get two 

 types divergent both from each other and from the 

 original A. 



In some such way as this it has appeared to many 

 biologists to be possible to explain the endless varieties 

 of related organisms that now cover the surface of the 

 globe and that peopled it in past ages, whose descen- 

 dants the former are. To Charles Darwin belongs 

 the credit of having been the first to clearly expound 

 the part played by natural selection in the evolution 

 of new forms in his great classic, " The Origin of 

 Species " (1859). To other biologists the theory 

 of natural selection has appeared more or less in- 

 adequate, even granting the genealogical relationship 

 of organisms, and many variations and modifications 

 of Darwin's theory have been promulgated during 

 the past half century. To one of these only can 

 reference be made here. 



One of the great objections offered to Darwin's 

 theory has been that the evolution of new forms by 

 natural selection would involve a quite stupendous 

 period of time ; and long, undoubtedly, as the earth 

 has been inhabited by plants and animals, even these 

 eons of time are considered inadequate for the evolu- 

 tion, by so slow a method, of the endless types of 

 organism that are now in existence or have existed 

 in past ages. Natura non facit saltus Nature does 

 not proceed by leaps has been an axiom with most 

 biologists since the days of Linnaeus, but during the 

 last few years, chiefly due to the persevering energy 



