( xcii ) 



employed " the word species as a designation for the 

 totah'ty of individuals differing from all others by marks 

 or characters which experience showed to be reasonably 

 constant and trustworthy, as is the practice of modern 

 naturalists." * 



This conception of a species is founded i;pon transition. 

 Whenever a set of individuals can be arranged, according to 

 the characters fixed upon by the systematist, in a series 

 without marked breaks, that set is regarded as a species. 

 The two ends of the series may differ immensely, may 

 diverge far more widely than the series itself does from other 

 series ; but the gradual transition proclaims it a single 

 species. If transitions were all equally perfect of course there 

 would be no difficu.lty. But tx^ansitions are infinite in their 

 variety ; while the subjective element is obviously dominant 

 in the selection of gaps just wide enough to constitute 

 interspecific breaks, just narrow enough to fuse the species 

 separated by some other writer, — dominant also in the choice 

 of the specific characters themselves. t Looking back upon the 

 interval between Linnaeus and Darwin, it seems remarkable 

 that the mutability of species was not forced upon systematists 

 as the result of their own labours. It is astonishing that 

 many a naturalist was not driven by his descriptive work to 

 the conclusion which Darwin stated to Asa Gray on July 20, 

 1856 : " — as an honest man, I must tell you that I have 

 come to the heterodox conclusion, that there are no such 

 things as independently created species — that species are only 

 strongly defined varieties." | 



For, as I have said above, every describer of species made 

 continuity and transition in characters the test of a variety, 

 discontinuity the test of a separate species. And in difficult 

 cases no two of them agreed in their conclusions. JNIany 

 passages in Darwin's correspondence convincingly prove how 

 essential an element is this continuity, and how inevitable 



* I. c. p. 370. 



t How important this choice may be is well shown hy Karl Jordan in 

 " Novitates Zoologica?," vol. iii, Dec. 1896, pp. 428-430. Characters are 

 subject to independent variation as well as correlated variation. Hence 

 there may he the widest discrepancy between the transitions constructed 

 by naturalists making use of different characters. 



J "Life and Letters," vol. ii, ji. 79. 



