148 PHILOSOPHY OF ZOOLOGY. 



I. Species. 



This term is universally employed to characterise a 

 group, consisting of individuals possessing the greatest 

 number of common properties, and producing, without con- 

 straint, a fertile progeny. 



The number of individuals belonging to a species, bears 

 no constant ratio to the numbers in other species, even of 

 the same genus. In the same species, the number is sub- 

 ject to remarkable variations, at diflPerent times. These 

 changes are either produced by alterations in the physical 

 circumstances of their station, or by the diminution or in- 

 crease of their foes, their food, or their shelter. 



Differences of character prevail, to a limited extent, 

 among individuals of the same species. These tend great- 

 ly to embarrass the student in his investigations, and 

 have led to the introduction of many spurious species into 

 the Systems of Zoology. Some of these differences neces- 

 sarily prevail, as the indications of the sex ; others are ac- 

 cidental, and constitute what are termed Varieties. 



A. Sexual Differences. — These occur, more or less, in 

 all those species in which the male oi-gans are seated on one 

 individual, and the female organs on another. In all cases, 

 the male is considered as the representative of the species. 

 While the female, in some species, differs remarkably from 

 the male in external characters, there is still an agreement 

 in structure, with the exception of the organs of the re- 

 productive system, and the modifications of some parts sub- 

 servient to their functions, When a female individual 

 comes under notice, it is frequently very difficult, if not 

 impossible, to determine the species to which she belongs, 

 where external characters alone are employed. This diffi- 

 culty, in the case of birds, meets the student at every step ; 



