CH. v] MUTATION 45 



extent were often so clear cut, and so distinct, that it seemed to 

 the writer quite evident that they must in general have been 

 formed by sudden change, or mutation. Gradual change, picking 

 out advantageous variation, would be very unlikely indeed 

 always to produce tlie same structural character, such, for example, 

 as is sho^vn by a berry or a drupe, or by opposite leaves. Why 

 should berries be most often found in the near (systematic) 

 neighbourhood of capsules, drupes in that of achenes or nuts? 

 Why should selection pick out leaves that were exactly opposite, 

 ovules with the raphe exactly dorsal or ventral, or why such 

 clearly marked and exactly formed fruits as capsules, berries, etc. ? 

 Selection would obviously act with decreasing force as the leaves 

 came nearer and nearer to being opposite (or alternate, for then 

 they show a definite phyllotaxy or arrangement), or the raphe 

 to being dorsal or ventral, etc. In actual fact, between many of 

 these characters, intermediate stages were not possible. One 

 could only take the one or the other side of a very divergent 

 variation, such as alternate or opposite leaves, dorsal or ventral 

 raphe, etc. The mutation, in so far as the characters themselves 

 were concerned, paid no attention to functional or adaptational 

 requirements. It was impossible to conceive of any adaptational 

 need that would ensure that all Monocotyledons should have a 

 single cotyledon, together with a parallel-veined leaf, a trimerous 

 flower, and a peculiar anatomy. There is not even a "mono- 

 cotyledonous " mode of life to which this great morphological 

 change might be supposed to adapt them. For that matter, 

 there is not even a " ranunculaceous " or a " thalictroid " mode of 

 life. The larger a genus, family, or other group of plants is, the 

 greater is the variety in the conditions of life in which it is found 

 (comparing, as usual, only related forms), and also the larger the 

 area covered by single individual species of the family or genus. 

 To return to the Dilleniaceae; assuming that mutations could 

 be of generic size, the author drew up a scheme according to 

 which the whole tree of the family could be looked upon as 

 derived by descent from a genus so comparatively simple in 

 structure, and so widely distributed, as Tetracera. A sketch was 

 drawn of a suggested manner in which the evolution might have 

 proceeded, showing all the existing genera, which might even be 

 the whole tree of the family. Of course some geological or other 

 catastrophe might have killed out some more or less local genera, 

 though it would be unlikely to have done so to any genus that 

 was already very widespread. Incidentally Tetracera is not the 



