522 



SCIENCE. 



[N. S. Vol. XXI. Xo. 536. 



steps as the more important process, if not 

 the only one efficient in the formation of 

 new species. Bateson ('94) has called 

 modifications of this sort discontinuous 

 variations, but de Vries (:01-:03) calls 

 them mutations, and the latter designation 

 seems likely to be generally adopted. 



Darwin rightly attached great impor- 

 tance to the variations of domesticated 

 animals and plants as throwing light on 

 the origin of species. He recognized that 

 there is no essential difference between 

 breeds and species, and that if we can as- 

 certain how breeds originate we can infer 

 much as to the origin of species. He made 

 an extensive study of breeds of animals as 

 well as of plants, but no one has followed 

 this up or even recognized its great impor- 

 tance until within very recent years. What 

 we need to know is how, precisely, are new 

 breeds formed. We know that they are 

 forming under our very eyes all the time 

 and that this has been going on since the 

 earliest historic times and no doubt a great 

 deal longer, yet the method eludes us. 



The successful practical breeder, the man 

 who originates breeds, is a keen observer, 

 a man of unusual intelligence and skill and 

 of infinite patience. Yet if we ask him 

 how, in general, he does his work, or how 

 a particular result was obtained, we rarely 

 get a satisfactory answer. This is some- 

 times because, for commercial reasons, it 

 is well to leave a cloud of obscurity sur- 

 rounding the origin of a successful breed, 

 lest its production be duplicated. More 

 often, however, it is because the breeder 

 himself does not know how the result was 

 attained. He may be able to tell us that 

 such and such animals were mated, such 

 and such of their offspring selected, and 

 after a certain length of time the breed was 

 established and put on the market. But 

 this, after all, gives us little information as 

 to the real nature of the material used and 

 the processes involved in the formation of 



the new breed. The aims of the biologist 

 are so different from those of the practical 

 breeder that to solve the theoretical prob- 

 lems involved in the formation of breeds 

 the biologist must himself turn breeder, and 

 see new organic forms arise out of material 

 with which he is thoroughly familiar, and 

 under conditions which he can control. So 

 little work of this kind has yet been done 

 that its fruits are scarcely ready to be 

 gathered. Generalizations can as yet be 

 made only tentatively, based on cases dan- 

 gerously few, or on the rather uncertain 

 and often contradictory testimony of prac- 

 tical breeders and the half-truths told by 

 stock registers. 



So far, however, as these various sorts of 

 evidence go, they indicate that the material 

 used by breeders for the formation of new 

 breeds consists almost exclusively of muta- 

 tions. The breeder does not set to work 

 with some purely imaginary form in mind, 

 toward which he seeks by selection gradu- 

 ally to mold his material. He commonly 

 either discovers the new breed already cre- 

 ated and represented by one or more excep- 

 tional individuals among his flock, or else 

 he seeks by cross-breeding to combine in a 

 single race characters which he finds al- 

 ready existing separately in different races. 

 In both cases he deals with mutations, i. e., 

 with characters unconnected by a series of 

 transition stages with the normal form. 

 An illustration from my own experience 

 may help to make this clear. A little more 

 than four years ago I obtained a number of 

 ordinary smooth-coated guinea-pigs and be- 

 gan breeding them with a particular ex- 

 periment in mind. Among nine young 

 produced by a certain pair, there was one 

 which had a supeimumerary fourth digit 

 on one of its hind feet. ISTeither of the 

 parents had such a digit, nor had I ever 

 heard of the existence of such a character 

 before, either in any of the wild Caviidae, 

 or among domesticated eavies or guinea- 



