830 TAXONOMY. 



moisture or dryness (which are transient and comparatively 

 unimportant), variation, or the unlikene.-s of progeny to parent, 

 is occult and inexplicable. If s< >m< 'limes called out l>v the 

 external conditions, it is by way of internal response to them. 

 In Darwin's conception, variation of itself does not tend in anv 

 Olie particular direction: he appears to attribute all adaptation 

 t<> the sorting which results from the struggle for existence and 

 the survival of the fittest. We have supposed, and N;cgdi takes 

 a similar view, 1 that each plant has an internal tendency or pre- 

 di-poMtion to vary in some directions rather than others : from 

 which, under natural selection, the actual differentiations and 

 adaptations have proceeded. I'nder this assumption, and taken 

 as a working hypothesis, the doctrine of the derivation of species 

 serves well for the co-ordination of all the facts in botany, and 

 affords a probable and reasonable answer to a long scries of 

 questions which without it are totally unanswerable. It is sup. 

 polled by vegetable paheontology. which assures us that the 

 plants of the later geological periods are the ancestors of the 

 actual flora of the world. In accordance with it we may explain, 

 in a good degree, the present distribution of species and other 

 groups over the world. It rationally connects the order of the 

 appearance of vegetable types in time with the grades of differ- 

 entiation and complexity, both proceeding from the simpler, or 

 lower and more general, to the higher and more differentiated 

 or special ; it explains by inheritance the existence of function- 

 less parts ; throws light upon the anomalies of parasitic plants in 

 their various gradations, upon the assumption of the most various 

 functions by morphologically identical organs, and indeed illumi- 

 nates the whole field of morphology with which this volume has 

 been occupied. It follows that species are not " simple curiosities 

 of nature," to be catalogued and described merely, but that they 

 have a history, the records of which an 1 impressed upon their 

 .structure as well as traceable in their geographical and paheon- 

 lological distribution. This view, moreover, explains the re- 

 markable fact that the characters in which the affinities of 

 plants are mainly discerned (and which therefore serve best 

 for orders, tribes, and other principal groups) are commonly such 

 as are evidently <>f small if any importance to the plant's well- 

 being. and that they run like threads through a scries of species 

 of the greatest diversify in habit, mode of life, and particular 

 adaptations to conditions." 



t,' mid Begriff dernaturhistorischen Art. Zweite Auflage, 1866. 



1 Tfiis is a corollary of natural .selection, which can take ell'ect only 

 U])on useful characters, i. <. upon structures which play sonic active part 



