CH. x] A. NUMERICAL 89 



We are still far from any understanding of the actual mechanism 

 of evolution, but if we can feel sure of the direction in which it 

 worked, we shall have made one step in advance which may open 

 a way to profitable lines of research. 



For example, take the case of economic botany, with its back- 

 ground of applied organic chemistry. So long as we imagine a 

 plant A, producing a valuable substance a;, to be descended from 

 some ancestor unknown, and quite probably unknowable, we are 

 heavily handicapped in tracing the origin and chemistry of x. 

 But if the descent, as differentiation would have it, were the 

 other wav, and the actual ancestors of A mav still be alive, so 

 that their chemistry may be studied, the work is greatly sim- 

 plified. Instead of remaining a vast mass of facts with little or 

 no co-ordination, economic botany may become a definitely 

 scientific subject, producing knowledge, not merely supplying it 

 in a dictionarv form, and we shall be able to look to valuable 

 results as yet quite unforeseen. 



Endemic or local plants, again, if they be regarded as usually 

 the youngest in their own circles of affinity, and therefore as 

 "the latest thing" in breeding, in chemistry, etc., may become of 

 great importance, instead of being regarded as practically 

 negligible relics, as at present. 



The writer hopes that the work here described may aid in 

 putting workers upon the right path towards a discovery of the 

 actual mechanism of evolution, and it seems to him that it may 

 be to cytology that we should look for the next step in advance. 

 As yet, the mutations that have appeared seem usually to be 

 lethal, recessive, or non-viable, but this is no proof that viable 

 or dominant mutations cannot appear also. If the result of 

 such a mutation were to be found growing anywhere, people 

 would at present say that it was another relic, and leave it at 

 that. Guppy has pointed out that many of the species that 

 have been found once only, and have never been seen again in 

 spite of search, are quite probably the result of such mutations, 

 which were in the early stages of establishing themselves, and 

 were perhaps exterminated by collecting specimens, or were not 

 viable (cf. 66, p. 151). 



As the two theories of the direction of evolution are diametri- 

 cally opposed, it seemed to the writer possible to devise some 

 crucial tests between them. A number of these have been thought 

 out from the principles laid down in Age and Area-, these sug- 

 gested others, which have led to more. This simple fact, that these 

 principles can be so extensively used for prediction, goes to show 



