no 



HYPOTHESIS OF THF DEVELOPMENT OF 



animals, the new being passes through stages in which it 

 is successively fish-like and reptile-like. But the resem- 

 blance is not to the adult fish or the adult reptile, but to 

 the fish and reptile at a certain point in their foetal pro- 

 gress ; this holds true with regard to the vascular, nervous, 

 and other systems alike. It may be illustrated by a sim- 

 ple diagram. The foetus of all the four classes may be 

 supposed to advance in an identical condition to the point 

 4. The fish there diverges and passes M 

 along a line apart, and peculiar to itself, 

 jo its mature state at F. The reptile, 

 «^ird, and mammal, go on together to C, -p. 

 where the reptile diverges in like man- 

 ner, and advances by itself to R. The 

 bird diverges at D, and goes on to B . 

 The mammal then goes forward in a 

 straight line to the highest point of or- 

 ganization at M. This diagram shows 

 only the main ramifications ; but the 

 reader must suppose minor ones, represent the subordi- 

 nate differences of orders, tribes, families, genera, &c, if 

 he wishes to extend his views to the whole varieties of 

 being in the animal kingdom. Limiting ourselves at pre- 

 sent to the outline afforded by this diagram, it is apparent 

 that the only thing required for an advance from one type 

 to another in the generative process is that, for example 

 the fish embryo should not diverge at A, but go on to C be- 

 fore it diverges, in which case the progeny will be not a fish 

 but a reptile. To protract the straightforward part of the 

 gestation over a small space — and from species to species 

 the space would be small indeed — is all that is necessary. 



This might be done by the force of certain external 

 conditions operating upon the parturient system. The 

 nature of these conditions we can only conjecture, for 

 their operation, which in the geological eras was so power- 

 ful, has in its main strength been long interrupted, and is 

 now perhaps only allowed to work in some of the lowest 

 departments of the organic world, or under extraordinary 

 casualties in some of the higher, and to these points the 

 attention of science has as yet been little directed. But 

 though this knowledge were never to be clearly attained, 

 it need not much affect the present argument, provided it 

 be satisfactorily shown that there must be some such in* 

 fiuence within the range of natural things. 



