June 22, 1906.] 



SCIENCE. 



949 



adjustment, and it is probably better to use 

 the latter word (or adaptation), if we want to 

 emphasize that these factors are empirical 

 facts; adjustment is a fact directly observed 

 in nature, while the struggle for life is an 

 inference drawn from other observations. 



I am prepared to accept this view in its 

 full meaning, namely, that we have to deal 

 here with facts, which may he observed in 

 nature, and the logical consequence of the 

 operation of these facts is evolution, that is 

 to say, the change of the organic world, or its 

 transmutation. But this does not exhaust all 

 the existing phenomena, for we observe in na- 

 ture a fourth fact, namely, that the chain of 

 organisms is cut up in species. This we may 

 call, with 0. F. Cook, speciation, and thus we 

 obtain altogether four facts: variation, in- 

 heritance, adjustment, speciation. These four 

 facts would satisfactorily explain the whole 

 of the organic world, if the causes of each of 

 them were known: the process of evolution, 

 consequently, is undeniable, and our investi- 

 gations should be conducted so as to discover 

 the causes of each of the main factors in 

 evolution. As we shall see presently, the dis- 

 cussion in evolution, and the difierences of 

 opinion have hinged chiefly upon this question 

 of the causes of these facts, and while in two 

 of them the causes are very clear, in the other 

 two they are much disputed. 



It is the chief shortcoming of some of the 

 modern writers, for instance Weismann and 

 de Vries, that they are oblivious of this fun- 

 damental idea of evolution, and the conse- 

 quence has been an utter confusion in their 

 views. For me it is simply past comprehen- 

 sion, how it was possible that the writings of 

 Weismann and de Vries have come to be 

 looked upon favorably, and to be regarded as 

 worthy of serious consideration. 



I have always regarded segregation (isola- 

 tion, separation) , as introduced by M. Wagner, 

 as the cau^e of speciation. This is, in my 

 opinion, the most vital improvement upon 

 Darwin's theory, and it is not opposed to it, 

 but rather an amendment or addition to it. 

 In this line, I have done some work myself, 

 chiefly by trying to show the real extent of 



the term segregation (Gulick). I shall not 

 go into detail here,' and only want to point 

 out that I consider speciation as fully ex- 

 plained by biological segregation. The latter 

 is a fact which, although it has not been 

 demonstrated in all cases, is now supported 

 by a sufficient number of actual observations, 

 and what is most important, a case that is 

 opposed to it has never been found, namely, 

 that two closely allied species occupy abso- 

 lutely the same range under identical ecolog- 

 ical conditions. Many other writers concur 

 with me on this point, and I name, aside from 

 M. Wagner, J. T. Gulick, G. Baur, D. S. Jor- 

 dan, J. A. Allen, C. H. Merriam. 



As the causes of adjustment, we are to re- 

 gard the struggle for existence and natural 

 selection consequent to it. Vaughan (L c, 

 p. 690) objects to the use of ' natural selec- 

 tion,' and possibly rightly so, considering how 

 this term has been abused, preeminently on 

 the part of Weismann. But the real value, 

 and the correct conception of natural selection 

 has been indicated by G. Pfeffer in a paper ' 

 which generally seems to have escaped atten- 

 tion. If we use natural selection in Pfeffer's 

 sense (not as the survival of the fittest, but as 

 the survival of fit individuals), I do not see 

 why this term should be objected to or dis- 

 carded. The struggle for life, which causes 

 natural selection, and consequently adaptation 

 or adjustment, is a logical deduction from 

 observations in nature, for we always see that 

 more individuals are produced than finally can 

 find place in the economy of nature. This 

 has been amply demonstrated by Darwin and 

 others, and thus the causes of adjustment are 



' See my publications : Grundziige der marinen 

 Tiergeographie, Jena, 1S96, p. 33. On Separa- 

 tion, and its bearing on Geology and Zoogeog- 

 raphy (Amer. Jonrn. Sci., 2, 1896, p. 63). On 

 Natural Selection and Separation {Pr. Amer. 

 Pliilos. Sac, 35, 1896, p. 182). Isolation as one 

 of the factors in Evolution (Science, January 12, 

 1906). A Case of Isolation without Barriers 

 (Science, March SO, 1906). 



' ' Die Umwandlung der Arten, ein Vorgang 

 funetioneller Selbstgestaltung ' ( Yerhandl. Natur- 

 loiss. Ver. Uawturg (3), 1, 1894). 



