724 THE PRINCIPLES OF SCIENCE. [CHAP. 



specified that the fruit is a pod, divided into two cells by 

 a thin partition, from which the valves generally separate 

 at maturity ; but we are also informed that, in a few genera, 

 the pod is one-celled, or indehiscent, or separates trans- 

 versely into several joints. 1 Now this must either mean 

 that the formation of the pod is not an essential point in 

 the definition of the family, or that there are several closely 

 associated families. 



The same holds true of typical classification. The type 

 itself is an individual, not a class, and no other object can 

 be exactly like the type. But as soon as we abstract the 

 individual peculiarities of the type and thus specify a 

 finite number of qualities in which other, objects may 

 resemble the type, we immediately constitute a class. If 

 some objects resemble the type in some points, and others 

 in other points, then each definite collection of points of 

 resemblance constitutes intensively a separate class. The 

 very notion of classification by types is in fact erroneous 

 in a logical point of view. The naturalist is constantly 

 occupied in endeavouring to mark out definite groups 

 of living forms, where the forms themselves do not in 

 many cases admit of such rigorous lines of demarcation. 

 A certain laxity of logical method is thus apt to creep in, 

 the only remedy for which will be the frank recognition of 

 ^he fact, that, according to the theory of hereditary descent, 

 gradation of characters is probably the rule, and 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 natural groups 

 constituting separate creations. Naturalists long held that 

 every plant belongs to some species, marked out by in- 

 variable 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, and hybrids, so that a species of plants 

 was often subdivided and classified within itself. But 

 then the differences upon which this sub-classification 



1 Bentham's Handbook of the British Flora (1866), p. 25. 



