A HISTORY OF SCIENCE 



one tree. For purposes of convenience of description, 

 we may divide organisms into orders, families, genera, 

 species, just as we divide a tree into root, trunk, 

 branches, twigs, leaves; but in the one case, as in the 

 other, the division is arbitrary and artificial. 



In Philosophic Zoologique (1809), Lamarck first ex- 

 plicitly formulated his ideas as to the transmutation of 

 species, though he had outlined them as early as 1801. 

 In this memorable publication not only did he state his 

 belief more explicitly and in fuller detail than the idea 

 had been expressed' by any predecessor, but he took 

 another long forward step, carrying him far beyond all 

 his forerunners except Darwin, in that he made an at- 

 tempt to explain the way in which the transmutation 

 of species had been brought about. The changes have 

 been wrought, he said, through the unceasing efforts of 

 each organism to meet the needs imposed upon it by 

 its environment. Constant striving means the constant 

 use of certain organs. Thus a bird running by the sea- 

 shore is constantly tempted to wade deeper and deeper 

 in pursuit of food ; its incessant efforts tend to develop 

 its legs, in accordance with the observed principle that 

 the use of any organ tends to strengthen and develop it. 

 But such slightly increased development of the legs is 

 transmitted to the offspring of the bird, which in turn 

 develops its already improved legs by its individual ef- 

 forts, and transmits the improved tendency. Genera- 

 tion after generation this is repeated, until the sum of 

 the infinitesimal variations, all in the same direction, 

 results in the production of the long-legged wading- 

 bird. In a similar way, through individual effort and 

 transmitted tendency, all the diversified organs of all 



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