CHAP. XI. EVOLUTION. 117 



As the conversion of the original development 

 with metamorphosis into direct development is here 

 under discussion, this may be the proper place to say a 

 word as to the already indicated absence of metamor- 

 phosis in fresh-water and terrestrial animals the 

 marine allies of which still undergo a transformation. 

 This circumstance seems to be explicable in two ways. 

 Either species without a metamorphosis migrated espe- 

 cially into the fresh waters, or the metamorphosis was 

 more rapidly got rid of in the emigrants than in their 

 fellows remaining in the sea. 



Animals without a metamorphosis would naturally 

 transfer themselves more easily to a new residence, as 

 they had only themselves and not at the same time 

 multifarious young forms to adapt to the new condi- 

 tions. But in the case of animals with a metamor- 

 phosis, the mortality among the larvae, always consider- 

 able, must have become still greater under new than 

 under accustomed conditions, every step towards the 

 simplification of the process of development must there- 

 fore have given them a still greater preponderance over 

 their fellows, and the effacing of the metamorphosis 

 must have gone on more rapidly. What has taken 

 place in each individual case, whether the species has 

 immigrated after it had lost the metamorphosis, or lost 

 the metamorphosis after its immigration, will not 

 always be easy to decide. When there are marine 

 allies without, or with only a slight metamorphosis, like 

 the Lobster as the cousin of the Cray-fish, we may 

 take up the former supposition; when allies with a 



