ORGANIC FORM AND ARCHITECTURE 709 



"But individuals truly exist in a different way; no one of tbem 

 exhibits at one time all the characteristics of the species. . . . 



"As representatives of Species, individual animals bear the 

 closest relations to one another; they exhibit definite relations also 

 to the surrounding elements, and their existence is limited within a 

 definite period." — Agassiz, 1859. 



"Species are merely those strongly marked races or local forms, 

 which when in contact do not intermix, and when inhabiting dis- 

 tinct areas are generally believed to have had a separate origin and 

 to be incapable of producing a fertile hybrid offspring." — Wallace. 



"Separate origin and distinction of race, evinced by a constant 

 transmission of some characteristic peculiarity of organisation, 

 constitutes a species."— Pritchard quoted by Wallace. 



"An assemblage of individuals which resemble each other in 

 their essential characters, are able directly or indirectly to produce 

 fertile individuals, and which do not (as far as human observation 

 goes) give rise to individuals which vary from the general type 

 through more than certain definite limits." 



"No two living beings are exactly alike, but it is a matter of 

 observation that among the endless diversities of living things, some 

 constantly resemble one another so closely that it is impossible to 

 draw any line of demarcation between them, while they differ only 

 in such characters as are associated with sex. Such as thus closely 

 resemble one another constitute a Morphological species."— HvxtEY, 

 1877. . 



But let us now consider this difficult question in more practical 

 mood. 



WHAT IS A SPECIES?— Most people know the stoat and the 

 weasel and could never confuse the one with the other; and these 

 are two nearly related species, which are often distinguished in 

 technical language as Mustela erminea and M. nivalis respec- 

 tively. They are two species of the genus Mustela, and one does 

 not require to be an expert observer to recognise that they are 

 nearer to one another than to an otter in the genus Lutra, or to a 

 badger in the genus Meles. Thus it is evident that a species is a group 

 of simDar individuals that have a good many characteristics in 

 common, yet are clearly distinguishable from other groups of similar 

 individuals. 



In the same way it is not difficult to distinguish the large raven 

 from the much smaller carrion crow, and both of them from the 

 rook, with its rough, bare patch round the base of the bill. Given 

 full-grown specimens, no one has any difficulty in distinguishing 

 these three species of the genus Corvus, the raven {Corvus corax), 

 the carrion crow (C. corone), and the rook (C. frugiUgiis). But a 

 little cloud of difficulty begins to form in the sky when we turn to 



