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THE AMERICAN NATURALIST [Vol. XLIX 



Obviously the environment can only be altered with 

 extreme slowness, and one of the first duties of the 

 parent is to provide for the developing offspring con- 

 ditions as nearly as possible identical with those under 

 which its own development took place. It is, however, 

 inevitable that, as phylogenetic evolution progresses, 

 the conditions under which the young organism develops 

 should change. In the first place, the mere tendency to 

 acceleration of development, to which we have already 

 referred, must tend to dislocate the correlation between 

 the ontogenetic series and the environmental series. 

 Something of this kind seems to have taken place in the 

 life-cycle of many hydrozoa, resulting in the suppression 

 of the free medusoid generation and the gradual degen- 

 eration of the gonophore. But it is probably in most 

 cases change in the environment of the adult that is re- 

 sponsible for such dislocation. 



To return to the case of the amphibians. At the 

 present day some amphibians, such as the newts and 

 frogs, still lay their eggs in water, while the closely re- 

 lated salamanders retain them in the oviducts until they 

 have developed into highly organized aquatic larva?, or 

 even what is practically the adult condition. Kammerer 

 has shown that the period at which the young are born 

 can be varied by changing the environment of the parent. 

 In the absence of water the normally aquatic larvas of 

 the spotted salamander may be retained in the oviduct 

 until they have lost their gills, and they are then born 

 in the fully-developed condition, while, conversely, the 

 alpine salamander, of which the young are normally born 

 in the fully-developed state, without gills, may be made 

 to deposit them prematurely in water in the larval, gill- 

 bearing condition. 



There can be no doubt that the ancestral amphibians 

 laid their eggs in water in a completely undeveloped con- 

 dition. The habit of retaining them in the body during 

 their development must have arisen very gradually in 

 the phylogenetic history of the salamanders, the period 



