330 TAXONOMY. 



moisture or dryness (which are transient and comparatively 

 unimportant) , variation, or the unlikeness of progeny to parent, 

 is occult and inexplicable. If sometimes called out by the 

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

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

 one particular direction : he appears to attribute all adaptation 

 to the sorting which results from the struggle for existence and 

 the survival of the fittest. We have supposed, and Naegeli takes 

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

 disposition to vary in some directions rather than others ; from 

 which, under natural selection, the actual differentiations and 

 adaptations have proceeded. Under 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 series of 

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

 ported by vegetable palaeontology, 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 are impressed upon their 

 structure as well as traceable in their geographical and palaeon- 

 tological 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 of small if any importance to the plant's well- 

 being, and that they run like threads through a series of species 

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

 adaptations to conditions. 2 



1 Entstehung und Begriff der naturhistorischen Art. Zweite Auflage, 1865. 



2 This is a corollary of natural selection, which can take effect only 

 upon useful characters, i. e. upon structures which play some active part 



