396 THE evoluliois of life. 



to tlie water now and then. Finally, if we ask under what 

 conditions this metamorphosis of a water-breather into an 

 air-breather completes itself, the answer is — it completes it- 

 self at the time when the shallow pools inhabited by the 

 larvsG, are being dried up by the summer^s sun.* 



See, then^ how significant are the facts when thus brought 

 together. There are particular habitats in which animals are 

 subject to changes of media. In such habitats exist animals 

 having, in various degrees, the power to live in both media, 

 consequent on various phases of transitional organization. 

 Near akin to these animals, there are some that, after passing 

 their early lives in the water, acquire more completely the 

 structures fitting them to live on land, to which they then 

 migrate. Lastly, we have closely-allied creatures like the 

 Surinam toad and the terrestrial salamander, which, though 

 they belong by their structures to the class Amphibia, are 

 not amphibious in their habits — creatures the larvae of which 

 do not pass their early lives in the water, and yet go through 

 these same metamorphoses ! Must we then think that the 

 distribution of kindred organisms through different media, 

 presents an insurmoimtable difficulty ? On the contrary, 

 with facts like these before us, the evolution-hypothesis 

 supplies possible interpretations of many phenomena that are 

 else unaccountable. Eealizing the way in which such changes 

 of media are in some cases gradually imposed by physical 

 conditions, and in other cases voluntarily coramenced and 

 slowly increased in the search after food ; we shall begin to 

 understand. how, in the course of evolution, there have arisen 



♦ While these pages are passing through the press, Dr Hooker has ohliged 

 me by pointing out, that *' plants afford many excellent examples" of analogous 

 transitions. He says that among true " water plants/' there are found, in the 

 same species, varieties which have some leaves submerged and some floating ; 

 other varieties in which they are all floating ; and other varieties in which they 

 are all submerged. .Further, that many plants characterized by floating leaves, 

 and which have all their leaves floating when they grow in deeper water, are 

 found with partly aerial leaves when they grow in shallower water ; and that 

 elsewb.ere they occur in almost dry soil with all their leaves aerial 



