XXJL.J CLASSIFICATION. 726 



depended were supposed to be variable, and thus dis- 

 tinguished from the invariable characters imposed upon the 

 whole species at its creation. Similarly a natural genus 

 was a group of species, and was marked out from other 

 genera by eternal differences of still greater importance. 



We now, however, perceive that the existence of any 

 such groups as genera and species is an arbitrary creation 

 of the naturalist's mind. All resemblances of plants are 

 natural so far as they express hereditary affinities ; but this 

 applies as well to the variations within the species as to 

 the species itself, or to the larger groups. All is a matter 

 of degree. The deeper differences between plants have 

 been produced by the differentiating action of circum- 

 stances during millions of years, so that it would naturally 

 require millions of years to undo this result, and prove 

 experimentally that the forms can be approximated again. 

 Sub-species may sometimes have arisen within historical 

 times, and varieties approaching to sub-species may often 

 be produced by the horticulturist in a few years. Such 

 varieties can easily be brought back to their original forms, 

 or, if placed in the original circumstances, will themselves 

 revert to those forms; but according to Darwin's views 

 all forms are capable of unlimited change, and it might 

 possibly be, unlimited reversion if suitable circumstances 

 and sufficient time be granted. 



Many fruitless attempts have been made to establish a 

 rigorous criterion of specific and generic difference, so that 

 these classes might have a definite value and rank in all 

 branches of biology. Linnreus adopted the view that the 

 species was to be defined as a distinct creation, saying, 1 

 " Species tot numeramus, quot diversae formse in principio 

 sunt creates ; " or again, " Species tot sunt, quot diversas 

 formas ab initio produxit Infinitum Ens ; qua? formse, 

 secundum generations inditas leges, produxere plures, at 

 sibi semper similes." Of genera he also says, 2 " Genus 

 onme est naturale, in primordio tale creatum." It was a 

 common doctrine added to and essential to that of distinct 

 creation that these species could not produce intermediate 

 and variable forms, so that we find Linnseus obliged by the 

 ascertained existence of hybrids to take a different view 



1 Philosophia Botanica (i77)> 1 57> P- 99- 



2 Ibid. 159, p. 100. 



