NEW TYPES AMONG ANIMALS 149 



of little differences. The tusk of the elephant, for example, prob- 

 ably did not reach its present size through innumerable tiny incre- 

 ments. More probably there occurred many larger mutations 

 whereby the tusks of a certain group of elephants were elongated 

 to such an extent that the group had an advantage over its short- 

 tusked comrades. 



What causes such mutations ? This is today the great question 

 of biology. Several causes are probably at work. Hybridization 

 is almost certainly an important factor. Two allied forms are 

 brought into contact, just as various races of men are brought 

 together in cities or in new countries. The two forms interbreed ; 

 there arises a hybrid form which is subject to wide variations; 

 and occasionally some unknown cause leads to extreme types. 

 These extreme types are genuine mutants, and give rise to new 

 species. When we have said that hybridization thus leads to new 

 species, we have not yet explained the matter. In the first place, 

 why do the two forms, hitherto separate, come together? Gener- 

 ally because of migrations. But among animals migrations are 

 almost invariably due to the search for food and water, or the 

 desire to escape enemies. The food and water depend almost 

 wholly on climate, while the number of enemies depends upon the 

 food supply of some other kind of plant or animal and thus again 

 upon climate, although less directly. Putting this aside, however, 

 and confining ourselves to what happens after the hybrid race is 

 formed, have we any evidence as to why the extreme mutants 

 arise.'* 



In attempting to explain why mutants arise, it must be remem- 

 bered that a mutant is different from a monstrosity which does 

 not reproduce itself. In order to have any great biological 

 significance a mutant must be reasonably perfect so that it can 

 reproduce its kind, and its peculiarities must be capable of being 

 passed on by heredity. Moreover, the mutation must occur in 

 enough individuals so that there is a reasonable chance that 

 animals with the same mutation will mate and thus perpetuate 



