CAUSES OF GENETIC VARIATION 233 



unusually compact proportions, the other of uniform intermediate 

 build, such is most commonly met with." It is to my mind 

 scarcely possible to resist the inference that, though there may be 

 definite responses to certain conditions, yet the chief distinctions 

 are genetic, and that it is these distinctions which confer the 

 power to respond. The parts respectively played by cause and 

 effect are always difficult to assign; but when it is stated that 

 "a weak-limbed, long-bodied and long- tailed animal becomes 

 well nigh perforce an undulatory swimmer, while the strong- 

 limbed, short-tailed, heavy-bodied specimen, when these charac- 

 teristics are rapidly forced upon it, is, under certain circum- 

 stances, just as forcibly induced to become a crawler," we feel 

 how erroneous any estimates of causation are likely to be. 



One of the most remarkable and interesting sections of 

 Powers' paper is that in which he describes the differences in 

 bodily structure and habits which he attributes to cannibalism, 

 and the whole account of the phenomena should be read in the 

 original. It appears that there are two extremely distinct 

 types of larvae, those with narrow heads and slender bodies 

 which live for the most part on small Crustacea such as Daphnias, 

 and those with huge mouths and very wide heads, which dis- 

 regard such small animals altogether and live on amphibian 

 larvae, whether of their own or other species. As the illustra- 

 tions show, the differences between these two types are very 

 great, and the differences in instinct and behaviour are no less. 

 The cannibals take no heed of the pelagic Crustacea, lying slug- 

 gishly at the bottom, rousing themselves immediately to a 

 violent attack on the larger living things which approach them. 

 Nothing but the most incontrovertible evidence based on abun- 

 dant control experiments should convince us that such differences 

 are not primarily genetic, and in the present state of knowledge 

 I incline to think that the families really consist of individuals 

 which are ready to assume the cannibal habit if opportunity 

 offers, and others which are congenitally incapable of it. It may 

 readily be that if all chance of cannibal diet be excluded, the 

 full development of the wide head and mouth, or the other 

 peculiarities, would never become pronounced, but I doubt whether 

 such change could be induced in any individual taken at random. 



