414 THE PRINCIPLES OF SCIENCE. 



the frank recognition of the fact that according to the 

 theory of hereditary descent, the gradation of characters 

 is probably the rule, and the precise demarcation between 

 groups the exception. , 



Natural Genera and Species. 



One important result of the establishment of the theory 

 of evolution, is to explode all notions about the existence 

 of natural groups constituting separate creations. Natura- 

 lists have long held that every plant belongs to some 

 species or group, marked out by invariable characters, 

 which do not change by difference of soil, climate, cross- 

 breeding, or other circumstances. They were unable to 

 deny the existence of such things as sub-species, varieties, 

 or hybrids, so that a species of plants was often sub- 

 divided and classified within itself. But then the dif- 

 ferences upon which this sub-classification depended were 

 supposed to be variable, and thus distinguished 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, in- 

 deed, are natural, so far as they express their hereditary 

 affinities, but this applies as well to the variations within 

 the species as to the species itself, or the larger natural 

 classes. All is a matter of degree. The deeper differences 

 between plants have been produced by the differentiating 

 action of circumstances 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 together again. Sub-species may often have 



