458 LECTUEE XVI. 



The case is altered when the changes, in consequence of 

 geological metamorphoses, are very slow. The adaptations 

 which these infinitesimal changes require, and which only- 

 become visible in their effects by the accumulations of thou- 

 sands of years ; these adaptations are as slight as the cause 

 which produces them, and they will consequently produce 

 such a number of gradual transitional forms that an infinite 

 number of specimens are required to connect the extreme 

 ends of the change. Do we not behold this in nature ? Do 

 we not see species of one group of strata slide into another; 

 that is to say, pass through a long series of development 

 stadia in order gradually to assume a form which differs from 

 the original form, not sufficiently, indeed, to be distinguished 

 under all circumstances, and yet enough to justify its distinc- 

 tion by the prefi:x: sub (e. g., Terebratula triquetra, and sub- 

 triquetra) from that found in another group of strata ? Have 

 we not seen the changes which, by the gradual elevation of 

 Sweden and Norway, have taken place in the Fauna of the 

 coasts ? Can we forget what Loven has shown, that by the 

 separation of the Wener- and Wetter-lakes from the sea with 

 which they were formerly connected, most species of this ice- 

 sea perished, but some craw-fish have preserved themselves in 

 these lakes gradually filled up with fresh water, and have so 

 adapted themselves to the changed medium, that though the 

 original type can be detected, peculiarities of form have been 

 developed establishing an essential transformation ? Does not 

 this example show us what all investigations concerning petri- 

 factions teach, that there exists nowhere a thorough separation 

 between two groups of strata, but that some individual species, 

 more or less in number, and more or less changed, pass from 

 one stratum into another ? 



We have seen that fixed species may alter under changed 

 conditions, and that this alteration of the surrounding media 

 is an essential lever for the production of those oscillating 

 types which pass under the name of raceless animals. We 

 have further learned that the fixed types interbreed less 

 readily the more constant that type. Is it not, hence, evident 

 that the productiou of new mongrel races must take place 



