

THE EXTINCTION OF SPECIES AND OF GROUPS 237 



have reached the maximum of power, either by 

 the dimensions of the body or by the perfection of 

 offensive or defensive weapons which would seem 

 to afford protection against all enemies, that these 

 species are on the eve of vanishing. All evolution 

 progressive in appearance, and all new adaptations, 

 are an extra danger for the survival of the type. 



Many palaeontologists have endeavoured, in the 

 course of the last few years, to examine still more 

 searchingly into the mechanism of the extinction 

 of species. As early as 1893, Dollo formulated, in 

 the concise form customary with him, the laws of 

 palaeontological evolution : development proceeds by 

 bounds, is irreversible, and limited. The first of 

 these propositions deals with the problem of the 

 formation of species, and will be the subject of dis- 

 cussion later on. The two other laws, that of 

 irreversibility and of limitation of development, 

 furnish precise statements of interest in the question 

 before us. By irreversible evolution is meant that 

 a branch, once started on the lines of a given 

 specialization, can, in no case, travel backwards 

 on the track traversed. Thus, the Horse, having 

 lost the lateral digits of its Tertiary ancestors or, 

 at least, transformed those metapods into two 

 osseous stylets buried in the flesh will never be 

 able to develop anew those rudimentary digits, 

 which must, on the contrary, tend more and more 

 to disappear. The Sirenians, descended, according 

 to all indications, from terrestrial Ungulates adapted 

 by degrees to aquatic life, whose hind limb has been 

 progressively reduced to an interior bony rod which 

 is the simple rudiment of the iliac bone, have now 



