720 THE PRINCIPLES OF SCIENCE. [CHAP. 



and plants the differences between individuals are slight 

 and apparently unimportant, while the numbers of such 

 individuals are immensely great, beyond all possibility of 

 separate treatment, scientific men have always stopped at 

 some convenient but arbitrary point, and have assumed 

 that forms so closely resembling each other as to present 

 no constant difference were all of one kind. They have, 

 in short, fixed their attention entirely upon the main 

 features of family difference. In the genealogical tree 

 which they have been unconsciously aiming to construct, 

 diverging lines meant races diverging in character, and 

 the purpose of all efforts at so-called natural classification 

 was to trace out the descents between existing groups of 

 plants or animals. 



Now it is evident that hereditary descent may have in 

 different cases produced very different results as regards 

 the problem of classification. In some cases the differ- 

 entiation of characters may have been very frequent, and 

 specimens of all the characters produced may have 

 been transmitted to the present time. A living fonn 

 will then have, as it were, an almost infinite number of 

 cousins of various degrees, and there will be an immense 

 number of forms finely graduated in their resemblances. 

 Exact and distinct classification will then be almost 

 impossible, and the wisest course will be not to attempt 

 arbitrarily to distinguish forms closely related in nature, 

 but to allow that there exist transitional forms of every 

 degree, to mark out if possible the extreme limits of the 

 family relationship, and perhaps to select the most 

 generalised form, or that which presents the greatest 

 number of close resemblances to others of the family, as 

 the type of the whole. 



Mr. Darwin, in his most interesting wcr-k upon Orchids, 

 points out that the tribe of Malaxese are distinguished from 

 Epidendrese by the absence of a caudicle to the pollinia ; 

 but as some of the Malaxese have a minute caudicle, the 

 division really breaks down in the most essential point. 

 " This is a misfortune," he remarks, 1 " which every natu- 

 ralist encounters in attempting to classify a largely 

 developed or so-called natural group, in which, relatively 



1 Darwin, Fertilisation of Orchids, p. 159. 



