THE CENTURY'S PROGRESS IN BIOLOGY 



only means, said Lamarck, that these similar groups 

 have had comparatively recent common ancestors, while 

 dissimilar sets of beings are more remotely related in 

 consanguinity. But trace back the lines of descent far 

 enough, and all will culminate in one original stock. 

 All forms of life whatsoever are modified descendants 

 of an original organism. From lowest to highest, then, 

 there is but one race, one species, just as all the mul- 

 titudinous branches and twigs from one root are but 

 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 Philosophie 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 an- 

 other long forward step, carrying him far beyond all his 

 forerunners except Darwin, in that he made an attempt 

 to explain the way in which the transmutation of spe- 

 cies 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 envi- 

 ronment. Constant striving means the constant use of 

 certain organs,'and such use leads to the development 

 of those 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. 



395 



