THE ORIGIN OF SPECIES. 61 



vival of the fittest.' He constantly compares the action of natural 

 selection to that of artificial selection, in which he supposes that the 

 breeder picks out those individual differences of the kind known as 

 ' fluctuating variations.' 



Modern zoologists who claim that the Darwinian theory is suffi- 

 ciently broad to include the idea of the survival of definite variations 

 seem inclined to forget that Darwin examined this possibility and 

 rejected it. The grounds for this rejection seemed valid at the time, 

 but a wider knowledge of the facts has shown that the problem is 

 simpler than Darwin was aware of. 



While Darwin uses the term ' struggle for existence ' in a very 

 loose, and often in a metaphorical sense, as he himself points out ; and 

 while it is true that he speaks of varieties and even species struggling 

 with each other, yet the central idea is that natural selection adapts 

 the organism to its environment by picking out and accumulating 

 slight, individual differences. It is, indeed, only in this way that 

 natural selection appears in the role of a creative factor in evolution, 

 and it is this power to build up new adapted types that appears to give 

 the theory its high value. 



Numerous cases of discontinuous variation have been known for a 

 long time, so that it is no mere assumption that such occur in nature. 

 Darwin himself has collected many instances of this sort, and amongst 

 domesticated animals and plants sudden variations have been frequently 

 recorded. In fact there can be no doubt, especially in the case of 

 plants, that such variations have often been utilized by the breeder, 

 even unconsciously at times. A few cases of this sort have been de- 

 scribed even for domesticated pigeons, and it is not improbable that the 

 great variety of domesticated breeds may, in part, have arisen in this 

 way, and not as the result of the selection of the individual fluctuations 

 of the wild rock-pigeon. Darwin argued that sudden variations can 

 not have been the source of new species, because, as a rule, when they 

 are crossed with the parent form the offspring are not intermediate, 

 but are exactly like one or the other parent, and it is a well known 

 fact that when wild species are crossed the hybrid is midway in char- 

 acter between its parents. Therefore, Darwin believed, wild species 

 could not have arisen as discontinuous variations. 



Our knowledge on these points has greatly increased since Darwin's 

 time, especially in recent years, mainly as the result of de Vries's work 

 on the evening primrose. It appears that there are several different 

 kinds of definite variations. The simplest cases are those in which 

 some one character suddenly becomes lost, as when an albino mouse 

 arises from a gray mouse. If such an albino is crossed with the parent 

 gray form the offspring in the first generation are all like the gray 

 mouse, but if these offspring are then inbred, they will produce some 



