48 CONTACTS WITH DARWINISM III [ch. v 



things much easier to understand is the widespread correlation 

 of characters, for which natural selection can offer no explanation. 

 Why is the possession of tendrils, or of hooked leaves or stems, 

 always accompanied by a weak and flexible stem? Why has a 

 dorsiventral leaf, such as is possessed by a vast number of plants, 

 always a layer of palisade tissue towards the upper side, for 

 making the best use of the light that falls upon it? Why, in the 

 Compositae, have the heads of flowers an involucre of bracts, 

 why has the style two stigmas, why is the ovary unilocular, why 

 is there only one ovule and that erect, and why is there no endo- 

 sperm? And why do all these characters go together in practically 

 every instance in a family of 18,000 species? The same sort of 

 questions may be asked for any other family, whilst they would 

 be absurd in the case of adaptational characters. Nothing but 

 descent from a common ancestor (or ancestors) will explain 

 them, and evolution upwards from individuals and varieties will 

 not do it; it must have been the other way, as differentiation 

 would have it. Evolution apparently must go on, at any rate if 

 the appropriate stimuli are present, but there is no necessary 

 adaptational reason for much of it, at any rate, and we find 

 practically no gradual stages in the fossil record. To accept 

 mutation, and that of any necessary size, would seem to be the 

 simplest theory upon which to work until something better 

 turn up. 



An objection often brought up is that no such mutations — 

 large, viable, not recessive, and not lethal — have been seen. But 

 no one has ever seen a species formed by natural selection. Yule 

 has estimated that one such mutation in fifteen to thirty years, 

 upon any small spot of the earth's surface, would be sufficient to 

 account for all the flowering plants that exist. The chance of 

 seeing such a mutation is all but non-existent, and if the result 

 were found at present, people would at once put it down as 

 another relic and leave it at that. Until we can control mutation 

 — and signs are not wanting that we may be able to do so at some 

 future time — we can hardly hope to get proof for this proposition. 



One must not forget that the mutations that have been studied 

 have, as a rule, been mutations that have occurred in cultivated 

 plants, or otherwise in unnatural conditions, conditions which in 

 themselves perhaps stimulated a greater mutability than usual. 

 We have not properly considered the case of mutations under 

 completely natural conditions, which are well kno^vn to be much 

 less common. If a mutation appear in a seedling of some tree in 



