58o THE BEGINNINGS OF LIFE. 



produced by the influence of altered conditions. And it 

 is often very difficult to make any distinction between 

 the two modes of change, owing to the almost imper- 

 ceptible gradations by which they tend to merge into 

 one another. 



What we have last said may be illustrated by 

 quoting a few of the examples cited by Mr. Darwin, in 

 which more or less marked changes in organisms have 

 been distinctly determined by some alteration of the 

 'conditions' to which they had been previously ex- 

 posed. Mr. Darwin says ' : — ^ With respect to the 

 common oyster, Mr. F. Buckland informs me that he 

 can generally distinguish the shells from different dis- 

 tricts, young oysters brought from Wales, and laid 

 down in beds where '^ natives " are indigenous, in the 

 short space of two months begin to assume the '' native " 

 character. M. Costa has recorded a much more 

 remarkable case of the same nature^ namely, that 

 young shells taken from the shores of England and 

 placed in the Mediterranean, at once altered their 

 manner of growth, and formed prominent diverging 

 rays, like those on the shells of the proper Mediter- 

 ranean oyster. The same individual shell, show- 

 ing both forms of growth, was exhibited before a 

 Society in Paris.' Other direct effects of change in 

 external conditions are perhaps brought about more 

 slowly ; thus, amongst many other examples, Mr. Darwin 



^ ' Animals and Plants under Domestication,' vol. ii. p. 280. 



