42 EMBRYOLOGY 



(Embryology.) This general unity of type in 

 great groups of organisms (including of course these 

 morphological cases) displays itself in a most striking 

 manner in the stages through which the foetus 

 passes \ In early stage, the wing of bat, hoof, hand, 

 paddle are not to be distinguished. At a still earlier 

 (stage) there is no difference between fish, bird, 

 &c. &c. and mammal. It is not that they cannot 

 be distinguished, but the arteries 2 (illegible). It is 

 not true that one passes through the form of a lower 

 group, though no doubt fish more nearly related to 

 foetal state 3 . 



This similarity at the earliest stage is remark- 

 ably shown in the course of the arteries which 

 become greatly altered, as foetus advances in life 

 and assumes the widely different course and number 

 which characterize full-grown fish and mammals. 

 How wonderful that in egg, in water or air, or in 

 womb of mother, artery 4 should run in same course. 



Light can be thrown on this by our theory. The 

 structure of each organism is chiefly adapted to the 

 sustension of its life, when full-grown, when it has 

 to feed itself and propagate 5 . The structure of a 

 kitten is quite in secondary degree adapted to its 

 habits, whilst fed by its mother's milk and prey. 

 Hence variation in the structure of the full-grown 

 species will chiefly determine the preservation of a 



1 See Origin, Ed. i. p. 439, vi. p. 605. 



2 In the Origin, Ed. i. p. 440, vi. p. 606, the author argues that the 

 "loop-like course of the arteries" in the vertebrate embryo has no 

 direct relation to the conditions of existence. 



3 The following passages are written across the page : " They pass 

 through the same phases, but some, generally called the higher groups, are 

 further metamorphosed. 



? Degradation and complication ? no tendency to perfection. 

 ? Justly argued against Lamarck ? " 



4 An almost identical passage occurs in the Origin, Ed. i. p. 440, vi. 

 p. 606. 



5 The following: "Deaths of brothers (when) old by same peculiar 

 disease " which is written between the lines seems to have been a memo- 

 randum which is expanded a few lines lower. I believe the case of the 

 brothers came from Dr R. W. Darwin. 



