1891.] ENTOMOLOGICAL NEWS. 47 



is the species* By a species of plants, animals, or insects, we 

 mean a group of individual plants, animals, or insects (i) alike 

 in appearance and (2) capable of producing fertile offspring 

 among themselves. f The first part (i) of this definition is mor- 

 phological, that is, it has to do with form, structure, etc. ; the 

 second part (2) is physiological, or has to do with functions or 

 actions. 



The morphological part of the definition, "alike in appear- 

 ance," admits of variations. It is well known that of very many 

 animals and plants there are varieties, differing more or less in 

 appearance, and that individuals of two different varieties, inter- 

 breeding, will produce fertile offspring. For this reason we must 

 consider these two varieties to belong to one and the same spe- 

 cies. On the other hand, two animals or plants, differing more 

 or less in appearance, which will not produce fertile offspring 

 between each other, must be considered to belong to different 

 species. 



Practically, in the study of insects, we can very rarely apply 

 the second or physiological part of our definition. We must, 

 therefore, rely to a very great extent on the morphological part.' 

 If we meet with two insects differing in structure, form, color, 

 etc., or any or all of these characteristics, we must consider them 

 as of different species, either until we learn of other insects in- 

 termediate between these two, and which " quite bridge over the 

 difference previously supposed to exist" between them, or until 

 we learn that these two insects, interbreeding, will produce fertile 

 offspring. But in applying these principles in our study, we 

 must be cautious about two things: in affirming the existence of 

 "new," that is, undescribed species, based on the existence of 

 single, more or less doubtful specimens; - - and in regarding 

 as of different species, the two dissimilar sexes of one and the 

 same species. 



But after having attempted thus much in explanation of what 

 a species is, it cannot be too strongly insisted that no hard and 

 fast lines exist in nature to separate species. Indeed, the desig- 

 nation of certain groups as "species," "genera," "orders," 



* Experience has shown that it is well to point out that the singular and the plural o 

 this word are spelled alike species. 



t In the preparation of this paper the writer is partly indebted to Prof. St. George 

 Mivart's work on " The Cat," chap. xii. iSSi. 



