GENERAL CONCLUSIONS. 803 



canine animal which preyed chiefly on rabbits, but sometimes on 

 hares, become slightly plastic : let these same changes cause the 

 number of rabbits very slowly to decrease, and the number of 

 hares to increase : the effect of this would be that the fox or dos; 



o 



would be driven to try to catch more hares ; his organisation, 

 however, being slightly plastic, those individuals with the lightest 

 forms, longest limbs, and best eyesight, let the differences be ever 

 so small, would be slightly favoured, and would tend to live 

 longer, and to survive during that time of the year when food was 

 scarcest ; they would also rear more young, which would tend to 

 inherit those slight peculiarities. The less fleet ones would be 

 rigidly destroyed. I can see no more reason to doubt that these 

 causes in a thousand generations would produce a marked effect, 

 and adapt the form of the fox or dog to the catching of hares 

 instead of rabbits, than that greyhounds can be improved by se- 

 lection and careful breeding.' l So Geoffrey Saint-Hilaire also 

 wrote : ( Si ces modifications amenent des effets nuisibles, les 

 animaux qui les eprouvent cessent d'exister, pour etre remplaces 

 par d'autres, avec des formes tin peu changees, et changees a la 

 convenance des nouvelles circonstances.' 2 



The modifications on which Geoffroy Saint-Hilaire laid chief 

 stress were those assumed to have affected the ambient medium, 

 the mode of operation of which in the origin of species he thus 

 exemplifies: ' Mon Memoire, traitant de Finfluence des milieux 

 ambians pour modifier les formes animales, montre comment la 

 quantite decroissante de 1'oxygene, relativement aux autres com- 

 posans de 1'atmosphere, a pu forcer les surfaces cutanees des 

 embryons, premier et principal siege des actes respiratoires, a 

 s'ouvrir davantage, a gagner, dans une raison inverse du volume 

 existant de 1'oxygene, plus de profondeur, au moyen de plus larges 

 anfractuosites dans le tissu cellulaire, et a acquerir, par im ac- 

 croissement dans 1'intensite des effets, de plus en plus, le carac- 

 tere d'ampoules et decidement de trachees, jusqu'a ce qu'enfin 

 survienne dans le thorax une concentration des sinus respiratoires, 

 et des arrangements de structure pour Fisolement des poches ou 

 theatres de respiration, appeles, suivant leurs qualites condition- 

 nelles, poumons on branchies.'' ccxcvn". p. 82. 



One should not be dealing fairly with this exposition of trans- 

 mutative conditions if we were to take its terms in their literal or 

 usual acceptation ; else, the obvious objection that embryos are 

 shut out from the influence of the atmosphere until their lungs 



1 ccci. p. 49. But see the remarks ou this in CLXXX. p. 434, and en', p. 65. 



2 ccxcix". p. 79. 



3 F 2 



