CH. XIV] GENERAL DISCUSSION 189 



family, genus, or other group, or such as climbing stems accom- 

 panied by the means of climbing, goes to show that the family 

 characters, or the climbing habit, must have been produced by 

 some sudden chromosomic change, but by what, and how, deter- 

 mined, we are as yet completely ignorant. Many other "adapta- 

 tions" come into the same categforv. 



A very probable large mutation, giving the ancestor of what is 

 now a large genus, is that which perhaps gave rise to the colum- 

 bine (Aquilegia), which can easily be imagined as arising from the 

 larkspur (Deljjhinium) by a mutation like that which often gives 

 a symmetrical sport in the toad-flax flower. 



It would seem probable that the early future development of the 

 study of evolution will be largely based upon the study of cytology, 

 for it would seem that the conception of gradual adaptation, at 

 any rate in its present form, must be abandoned. The larger 

 groups seem to have appeared before the smaller, upon the whole, 

 the force or size (if one may use such a term) of the mutations 

 that went on diminishing as time went on, the number of smaller 

 mutations on the whole increasing in proportion to the larger. 

 What actual part the external conditions took in the matter 

 is at present inexplicable, but there is nothing in the structural 

 characters, as a general rule, to show that the part was a large one. 



One must not lose sight of the hybridisation that is so easily 

 possible, and of which Lotsy (27) made so important a feature in 

 evolution. At the same time, if mutation can take place, as 

 seems highly probable, in such a way as to cross the "sterility 

 line" between species, and so to isolate them, it does not seem 

 very likely that fertile species -hybrids will be produced in such 

 numbers as to have an important influence upon evolution 

 generally, though one must not forget the possible influence 

 of the cosmic rays or other factors in causing the doubling of the 

 chromosome numbers. Hybridisation seems very unlikely among 

 the widely separated genera that seem to be the firstcomers, in 

 most, if not in all, families, but as one goes down the scale, one 

 seems to come among genera that are closer and closer together 

 in their taxonomic characters, and with these hvbridisation 

 would seem to become more and more possible, and more likely 

 to occur. Still more would this be the case among the species, 

 and here again rather in the species of large genera. It seems to 

 the writer that this question of hybridisation, with its increasing 

 possibilities in the genera and species of later formation, may be 

 one of some importance, though one must, of course, not forget 



