4H THE POPULAR SCIENCE MONTHLY. 



light the tenuity of the distinction between species and varieties. 

 The fact of embryology, the occurrence of rudimentary organs, and 

 the fundamental unity of structure which obtains in vast groups, 

 such as the vertebrata and arthropoda, all tended to suggest the ex- 

 istence of a genetic connection between species, so that Lamarck was 

 finally led to renounce the doctrine of the fixity of species, and to 

 define a species as " a collection of individuals which resemble each 

 other and produce their like by generation, so long as the surrounding 

 conditions do not alter to such an extent as to cause their habits, 

 characters, and forms, to vary." 



According to this definition the distinction between species and 

 variety once more becomes conventional. A variety is, in fact, a nas- 

 cent species ; and the notion of the creation of species vanishes, inas- 

 much as every species is the result of the modification of a predeces- 

 sor. Lamarck's views of the nature of geological changes were in 

 harmony with his biological speculations, and wholesale catastrophic 

 revolutions were as completely excluded from the one as from the 

 other. 



It is impossible to read the " Discours sur les Revolutions " of 

 Cuvier, and the " Principes " of Lamarck, without being struck with 

 the superiority of the former in sobriety of thought, precision of 

 statement, and coolness of judgment. And it is no less impossible to 

 consider the present state of biological science without being im- 

 pressed by the circumstance that it is the conception of Lamarck 

 which has triumphed,' and that of Cuvier which has been utterly van- 

 quished. 



Catastrophic geology has vanished out of sight, and is everywhere 

 replaced by the conception of slow and gradual change. With it has 

 disappeared the once prevalent notion that the whole living popula- 

 tion of the earth has been swept away and replaced in successive 

 epochs. On the contrary, it is now well established that the changes 

 which have taken place in that population have been effected by the 

 slow and gradual substitution of species for species. 



Moreover, it is well established that in some cases the succession 

 of forms in time is the same as that which should have occurred if 

 the hypothesis of evolution is correct. 



The rapid advance of comparative anatomy has diminished or re- 

 moved the wide intervals which formerly appeared to separate the 

 different divisions of the animal and vegetable kingdoms from one 

 another. Even the hiatus between the vertebrata and the inverte- 

 brata is bridged over by recent discovery. The establishment of the 

 cell-theory, however much the views originally propounded by 

 Schwann have been modified, leaves no doubt that there is a funda- 

 mental similarity in minute structure, not only between all animals, 

 but between them and plants, while the discoveries of embryologists 

 have proved that even the most complex forms of living beings do, in 



