or HEAT ON DEVELOPMENT, 129 



fig. 33) and Branchipus, can endure much greater variations 

 and develope equally well at any point between zero and 30°. 

 The difference between the extremes of temperature which, on 

 the whole, favour the development of these eurythermal eggs 

 (having, that is, a wide range of temperature) is apparently even 

 much greater, although even with them the rapidity of their 

 development depends on a rapid rise of temperature to a tole- 

 rably high degree.^' Thus I myself have observed the larvse of 

 Branchipus and Apus, known as Nauplius, hatched out in 

 less than twenty-four hoxirs at a temperature of 30° cent., but 

 at 16°-20° they required some weeks. The same effects of 

 vai-iation of temperature were observed by Higginbottom in 

 the larvae of frogs; he found that at a temperature of 51° F. 

 ( = 10'5° cent.) they hatched in twenty-one' days, but at 60° F. 

 (=15'5° cent.) they hatched in ten days. Hence a rise in tem- 

 perature of only 9° F. (=5° cent.) doubled the rapidity of 

 development of these embryos. The difference is still more 

 conspicuous if we compare the time required in the two cases 

 for the whole process of development from the time the egg was 

 laid till the metamorphosis was complete. At the lower tem- 

 perature of 51° F. 235 days were needed, at the higher, 60° F., 

 only seventy-three days. Many other examples might be added 

 to these, all proving the same effect of a rising temperature ; 

 but unfortunately, so far as I know, none give any exactly deter- 

 mined thermal curve for particular species, and all that is clearly 

 shown by these observations, often merely incidentally made, 

 is the fact that the eggs of different, very closely related species 

 behave in a totally dissimilar manner. An increase of tem- 

 perature in a district, occasioned by no matter what cosmical 

 causes, may thus affect some animals particularly favourably, 

 while it injures others, since for the former it may merely raise 

 it to the optimum, while for the latter it exceeds the maximum 

 they can on the whole endure. In this way, by acting on the 

 sexual activity and the rapidity of growth in the young animals 

 and in the embryo, a transforming influence may be closely 

 connected with the selective influence exerted upon the various 

 spgcies generally. It would certainly be an interesting and 

 valuable task to investigate this point with accuracy. 



