24 MANUAL OF ZOOLOGY. 



factory element in the definition of " species/' The sterility, 

 however, of hybrids is not universal, even amongst the higher 

 animals ; and amongst plants no doubt can be entertained but 

 that the individuals of species universally admitted to be dis- 

 tinct are capable of mutual fertilisation ; the hybrid progeny 

 thus produced being likewise fertile, and capable of reproduc- 

 ing similar individuals. That this fertility is often irregular, 

 and may be destroyed in a few generations, admits of explana- 

 tion, and hardly alters the significance of these undoubted facts. 



Upon the whole, then, it seems in the meanwhile safest to 

 adopt a definition of species which implies no theory, and 

 does not include the belief that the term necessarily expresses 

 a fixed and permanent quantity. Species, therefore, may be 

 defined as an assemblage of individuals which resemble each other 

 in their essential characters, are able, directly or indirectly, to pro- 

 duce 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. The pro- 

 duction of occasional monstrosities does not, of course, invali- 

 date this definition. 



Genus is a term applied to groups of species which possess 

 a community of essential details of structure. A genus may 

 include a single species only, in cases where the combination 

 of characters which make up the species are so peculiar that 

 no other species exhibits similar structural characters ; or, on 

 the other hand, it may contain many hundreds of species. 



Families are groups of genera which agree in their general 

 characters. According to Agassiz, they are divisions founded 

 upon peculiarities of "form as determined by structure." 



Orders are groups of families related to one another by 

 structural characters common to all. 



Classes are larger divisions, comprising animals which are 

 formed upon the same fundamental plan of structure, but differ 

 in the method in which the plan is executed (Agassiz). 



Sub-kingdoms are the primary divisions of the animal king- 

 dom, which include all those animals which are formed upon 

 the same structural or morphological type, irrespective of the 

 degree to which specialisation of function may be carried. 



Impossibility of a Linear Classification. It has sometimes 

 been thought that the animal kingdom can be arranged in a 

 linear series, every member of the series being higher in point 

 of organisation than the one below it. As we have seen, how- 

 ever, the status of any given animal depends upon two condi- 

 tions one its morphological type, the other the degree to 

 which specialisation of function is carried. Now, if we take 



