30 PLANT-ANIMALS [CH. 



species occurs in dry alpine regions of relatively low 

 temperature ; the latter, in lower regions with plenty 

 of water and of higher temperature. 



In the dry, alpine regions S. atra deposits eggs 

 which hatch out as land-animals ; in the wet lowlands, 

 the eggs laid by S. maculosa contain embryos in a 

 less advanced stage of development. The young, 

 when born, are gill-bearing and complete their de- 

 velopment whilst leading an aquatic mode of life. 

 Thus each species is adapted to the physical con- 

 ditions of its environment. 



But it has been shown that if S. atra is exposed 

 to lowland conditions, that is, to a moist atmosphere 

 and a relatively high temperature, it lays its eggs 

 earlier, the young hatch out in the gill-bearing stage 

 and development is completed during their life 

 as independent, aquatic animals. Conversely, if 

 S. maculosa is exposed to alpine conditions, oviposition 

 does not take place till the embryos have passed 

 beyond the aquatic, gill-bearing phase. Therefore, 

 in these circumstances, they are born as land- 

 animals. 



Hence the adjustment of each species to its 

 environment is due to the direct effect of certain of 

 the physical conditions of that environment on the 

 course of development of the embryos. The fact of 

 adaptation is not denied, but the mechanism whereby 

 it is effected is discovered, and the way made clear 



