D.— ZOOLOGY 89 



and so may that previously mentioned (p. 83), adjusting a plant to another 

 rhythm of light and darkness. Doubtless such potentially pre-adaptive 

 mutations are not uncommon, and may play an important role in widely 

 dispersed types, and during periods of changing environment. 



That selection can influence adaptive characters is shown by a number 

 of lines of evidence, experimental as well as indirect. Cesnola found 

 experimentally that the colours of Mantids exerted a protective effect in 

 relation to enemy attacks. We have already mentioned the results of 

 Heslop Harrison on the colours of certain moths. 



Then there is now a large body of experimental evidence showing that 

 insects with warning colours are on the whole rejected, while those with 

 protective colours are on the whole accepted. One of the most interesting 

 pieces of evidence as to the efficacy of selection in maintaining mimetic 

 adaptation is afforded by unpublished data for which I am indebted to 

 Mr. E. B. Ford. The butterfly Papilio dardanus has several mimetic 

 types of female. Random collections were made from two areas. In 

 one of these the models were far more numerous than the mimics, while in 

 the other, on the limit of the models' range, the models were actually less 

 abundant ; the actual ratios were 17-6 : 1 and 0-24 : 1. The collections 

 showed that whereas in the former case the mimetic resemblance was very 

 close (mimics classified as imperfect being below 4 per cent.), in the latter 

 it was far from exact (31*5 per cent, of imperfect mimics), and the varia- 

 bility of the mimics much greater. 



The evidence that we possess goes to show, first, that selection can be 

 very efficacious in altering the mean of a population within the range of 

 existing variability ; secondly, that a relaxation of selection will allow 

 the type to deviate away from adaptive perfection, quite outside the range 

 of variability to be found where selection is more stringent ; and, thirdly, 

 that adaptive characters may advantage their possessors in such a way 

 as to exert definite selection- pressure in their favour, and that accordingly 

 selection can have a continuous guiding effect towards adaptive perfection. 



Some Fallacies. 



Here we must turn aside to consider long-range evolutionary trends. 

 It is quite clear that many of these are adaptive. So obvious is this fact 

 that it has found expression in the current phrase adaptive radiation. 

 When palaeontological evidence is available the adaptive radiation is 

 seen to be the result of a numebr of evolutionary trends, each tending 

 to greater specialisation — in other words, to greater adaptive efficiency 

 in various mechanisms subservient to some particular mode of life. 

 Specialisation continues steadily for a considerable time, which in the 

 higher mammals at least seems to last between ten and forty million years ; 

 eventually change ceases, and the specialised type either rapidly becomes 

 extinct or else continues unchanged for further geological periods. 



It is hard to understand why the trends seen in adaptive radiation 

 have been adduced as proof of internally determined orthogenesis. 

 Whenever they lead to improvement in the mechanical or neural basis 

 for some particular mode of life, they will confer advantage on their 

 possessors and will come under the influence of selection ; and the selec- 



