30 MANUAL OF ZOOLOGY. 



the higher animals, " crosses " or " hybrids " can only be pro- 

 duced between closely-allied species, and when produced they 

 are sterile, and are not capable of reproducing their like. In 

 these cases, therefore, we may take this as a most satisfactory 

 element in the definition of " species. 5 ' 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 distinct 

 are capable of mutual fertilisation ; the hybrid progeny thus 

 produced being likewise fertile, and capable of reproducing 

 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 



