LECTURE VIII. 217 



common characters^ which mark them as actual or possible 

 descendants from the same parent stocky belong to the same 

 species. 



Let us for the present adopt this definition and keep to 

 observation, which^ in most cases, is unable to trace actual 

 descent, but must attend to the characters of the individuals. 

 We find in a certain region a type of animals, — a good species, 

 as naturahsts would call it, — which is easily recognised ; we 

 coUect a number of such individuals, we dissect them, we ob- 

 serve their development, compare them so as to obtain a per- 

 fect knowledge of the species. The question arises now, is 

 the type we have so closely studied universal and unalterable ? 



Observation teaches that this question must be answered in 

 the negative. We find almost all naturalists agreeing that 

 species has a wide range, in which the characters of the indi- 

 viduals composing it are changed. In all books and treatises, 

 varieties are mentioned which are subordinate to species. But 

 as regards the term variety, and the notion to be attached to 

 it, its limits and relation to species, the opinions are conflict- 

 ing ; some authors calling that a variety which others term a 

 species. Linngeus defined variety, as any alteration produced by 

 an accidental cause ; Geofi'roy St. Hilaire terms a variety a 

 simple anomaly which does not obstruct the performance of 

 any function. We may, perhaps, ask what is the hmit of this 

 simple deviation ; and as no general rule can be given, it will 

 depend on the judgment of the observer what limits he assigns 

 to variety. We find, in point of fact, that each type and each 

 species has, in this respect, its own laws ; that a deviation 

 which is insignificant in the one may be great in another. It 

 is, consequently, very difiicult to lay down a general definition 

 of variety, the more so as an accidental abnormity may become 

 normal by its permanence. Let us examine this subject. By 

 some accidental influence a short-legged ram is produced in a 

 long-legged flock. This is an accidental abnormity confined 

 to one individual, — the case is an exceptional one, and the 

 naturalist does not look upon this individual as constituting a 

 variety. But this ram has descendants ; the local conditions, 

 let us assume, favour the propagation of short-legged sheep 



