SUMMARY OF'PHILOSOPHIE ZOOLOGIQUE? 291 



in its changes assumes forms which are not allied with 

 any others by steps that we can recognize. An organ 

 may disappear and reappear several times before being 

 entirely lost : but this is what we might expect, for the 

 cause which has led to the evolution of living organisms 

 has evolved many varieties, due to external influences. 

 Nevertheless, looking at organization broadly, we 

 observe a descending scale." * 



" If the tendency to progressive development was the 

 only cause which had influenced the forms and organs 

 of animals, development would have been regular 

 throughout the animal chain ; but it has not been so : 

 Nature is compelled to submit her productions to an 

 environment which acts upon them, and variation in 

 environment will induce variation in organism : this is 

 the true cause of the sometimes strange deviations from 

 the direct line of progression which we shall have to 

 observe. t 



" If Nature had only called aquatic beings into 

 existence, and if these beings had lived always in the 

 same climate, in the same kind of water, and at the 

 same depth, the organization of these animals would 

 doubtless have presented an even and regular scale of 

 development. But there has been fresh water, salt 

 water, running and stagnant water, warm and cold 

 climates, an infinite variety of depth : animals exposed 

 to these and other differences in their surroundings 

 have varied in accordance with them, f In like manner 

 those animals which have been gradually fitted for 

 living in air instead of water have been subjected to 

 * 'Phil. ZooL,' torn. i. p. 143. f Page 144. J Ibid. 



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