122 THE INFLUENCE OF INANIMATE SUBEOUNDINaS. 



drought whicli prevails during the hot season is the direct 

 cause of itj as will be more fully shown in a future chapter. 



One more effect of rising temperature must here be men- 

 tioned as influencing one particular function, on whose normal 

 action the existence of every species primarily depends — the 

 multiplication of individuals, or reproduction, either by eggs, or 

 by fission and budding. Every modification of this function 

 must exert a direct and considerable influence, either selective 

 or transforming, on every separate species. If, for instance, we 

 suppose that the animal temperature in any country were to be 

 so modified that the optimum temperature for the production 

 of eggs did not occiu- at the proper season nor last a sufficiently 

 long time, all the animals affected by the change would be 

 infallibly exterminated, and the fauna would be utterly trans- 

 formed at a single blow. The influence of a rising temperature on 

 the eggs already laid, and about to develope after their winter rest, 

 would be almost equally important, as affecting the composition 

 of the fauna of a country. Before going into particulars, it will 

 perhaps be well once more to remind the reader that by the 

 words ' rising temperature ' I mean, not an increase of heat 

 from a fixed minimum to a certain uniform maximum, but a 

 rise from any degree above zero to any other, the optimum — 

 two points which, as we have seen, have no direct reference to 

 the degree of temperature, but only to the mode in which they 

 may affect animals. 



It seems to me superfluous to addiice any special instances 

 in support of the weU-known statement that egg-formation 

 undoubtedly begins — in other words, the sexual powers attain 

 maturity — under a rising temperature. No experiments, how- 

 ever, have hitherto proved how this effect is produced by heat, 

 whether heat alone can have that ■effect, or whether it requires 

 other concurrent circumstances ; nor at what degree of rising 

 temperature the result is produced in different animals. Several 

 observations accidentally made^^ appear to be most happily 

 explained' by this hypothesis of the effect of warmth, but no 

 attempt has been made, on any comprehensive method at any 

 rate, to supply an exact answer to the questions to which those 

 very observations first gave rise. Thus, for instance, and to go 



