52 THE WANDERINGS OF ANIMALS [CH. 



to hibernate ; even during the cold nights of their 

 summer the cold-blooded kinds become lethargic. In 

 conformity herewith we find that specimens of the 

 same kind, which in the warmer zones live an all-the- 

 y ear-round life, have learned to hibernate in the upper 

 levels. High up there under the very tropics, but at 

 10,000 feet elevation, the summer is so short that 



7 / 



stationary creatures like lizards and snakes lead only 

 a kind of one-quarter life. This implies that, for 

 instance, a four-year-old snake has really lived, that 

 means to say eaten, grown and loved, only for twelve 

 months. The result is that many a high -mountain 

 fauna contains not a few small-sized species in com- 

 parison with their more favourably placed relations 

 lower down. Another result seems to be that many 

 cold-blooded vertebrates have given up the laying 

 of eggs which might not be hatched on the clammy 

 ground, and have become viviparous, undoubtedly a 

 safer mode of looking after the welfare of their kind. 



The present writer has made a special study of 

 altitudinal distribution in Mexico. It remains to be 

 seen whether the following conclusions apply to other 

 countries, and therefore are widely applicable general 

 laws. 



1. Hot land species ascending into cooler zones 

 are liable to lose their specific characters and to 

 assume others ; that means to say they change into 

 new species. 



