484 THE EVOLUTION OP LIFE. 



quits the water only occasionally; or, like the Frog, one 

 which pursues a life mainly terrestrial, and returns to the 

 water now and then. Finally, if we ask under what condi- 

 tions this metamorphosis of a water-breather into an air- 

 breather completes itself, the answer is — it completes itself 

 at the time when the shallow pools inhabited by the larvae 

 are being dried up, or in danger of 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 closel3^-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 larvas of which 

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

 these same metamorphoses ! Must we then think, like 

 Von Baer, that the distribution of kindred organisms through 

 different media presents an insurmountable difficulty? On 

 the contrary, with facts like these before us, the evolution- 

 hypothesis supplies possible interpretations of many phe- 

 nomena that are else unaccountable. After seeing the ways 

 in which such changes of media are in some cases gradually 



* While these pages are passing through the press (in 1864), Dr. Hooker 

 has obliged 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 whicli 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 elsewhere they occur in almost dry soil with all 

 their leaves aerial. 



