LECTURE XVI. 459 



at that period when, by the change of surrounding media, the 

 fixity of the type is broken and produces that raceless soil 

 from which the various types spring up, partly by inter- 

 mixture and partly by adaptation, in order to become again 

 fixed types ? 



It appears to me that in this way may be explained both the 

 renewal of creation in difi'erent epochs, as well as the extinc- 

 tion of most species at the same periods, and also the fixity of 

 types during long periods intervening between renovation 

 epochs, and further the development of more perfect types 

 from the raceless masses which arise in the beginning of the 

 renovation period. There may, in many respects, be progress, 

 arrest, or retrogression. Thus the type of the Ammonites seems 

 to us a more perfect type than that of the Nautilus, neverthe- 

 less the former became extinct at the end of the chalk period. 

 The cavern-bear was more of a beast of prey than his de- 

 scendant the brown bear, and this can hardly be called a 

 progress. 



Acquainted as we are with the retrogressive metamorphosis 

 in animals, namely, how an animal may in its earliest youth 

 have a more perfect structure than at a later age, why may we 

 not imagine a similar process taking place during the adapta- 

 tion to changed conditions which no longer permit the type 

 to continue in its former perfection ? Why should not 

 types become gradually fixed by their adaptation to changes, 

 and modify their sensitive and locomotive apparatus, which 

 can no longer be used as formerly, when they moved under 

 different conditions, by which their senses and limbs acquired 

 a certain perfection ? Modifications of this kind which, from 

 an anatomical standpoint, must be considered as a retrogres- 

 sion, may, under given circumstances, be as advantageous in 

 the struggle for existence as the transformation of the palmated 

 feet of the larvae of certain parasitic Crustacea into hooks and 

 claws is advantageous for their subsequent, so to speak, seden- 

 tary life. 



But whilst we thus proceed hand in hand with observation, 

 we must not forget that transformation by adaptation, or by 

 intermixture, is still confined within certain limits which are 



