RECAPITULATION. 491 



always associated with differences in the conditions of Hfe. 

 If the conditions of Hfe have nothing^ to do with the origin 

 of variations then the variations of the fins in the two stages 

 of the Amphibian will be the same as in the Dipnoan which 

 lives under nearly the same conditions all its life. Only the 

 standard of selection will be different. But it is impossible 

 to believe that the selection of individual variations such as 

 occur in the Dipnoans could produce a form in which there 

 were two stages of life, one without limbs and the other 

 with well-developed pentadactyle limbs. There is no evi- 

 dence that such a thing is possible. 



On the other hand we have plenty of evidence that 

 conditions of life acting temporarily during a particular 

 period of life produce definite changes which disappear 

 when different conditions act upon the same individual. 

 Muscles and skeletal structures diminish when disused and 

 increase when exercised. As far as the individual is con- 

 cerned, apart from inheritance, we know that adaptation of 

 structure to different conditions of life at different periods 

 of life actually does take place. It cannot be denied that 

 the jDrocess of evolution so far as we are able to elucidate 

 it is such as would result if such changes as are produced 

 by conditions in the individual were inherited, if successive 

 states corresponding to successive modes of life tended to 

 recur in every generation, so that the effects of conditions 

 were cumulative. The Dipnoan ancestor of the Amphibian 

 was able to respire air by mt^ans of its air-bladder. Reaching 

 places where the air was moist, but water in any large 

 bodies scarce in summer, it would be forced to depend more 

 on air breathing, and less on its gills. The latter would 

 be more or less dried and, therefore, undergo retrogressive 

 changes. The air-breathing adult would deposit its eggs 

 in small bodies of water where want of room, the persecu- 

 tion of enemies, and the scarcity of animal food would force 

 the larva to live among water-weeds and live upon them. 

 It would cease to use its paired fins, and cease to exercise 

 the muscles and rays of the median fins. The larva would, 

 therefore, on Lamarckian views, become degenerate. Ac- 

 cording to those views, development of organs depends 



33 



