xxiv Introduction. 



The above-quoted saying of Mr. Macleay's, by suggesting the 

 question, ' When is a character to be considered as proved to be 

 natural?' brings us face to face with the most distinctive pecu- 

 liarity of zoological classification. A character is a good basis for 

 classification in zoology, as in every other subject, when its presence 

 enables us to predict the presence of many, or at least of some 

 other characters besides those which its name implies etymolo- 

 gically ; but when we are concerned with species in zoology, these 

 other characters must relate not only to the entirety of the or- 

 ganism as such, but also to the main facts of its life histoiy. 

 When we class two living organisms together in the same species, 

 we include always among the other facts which their common 

 specific name must connote, the particular fact that it is possible 

 for them both to have descended from one ancestor or ancestors, 

 which, either directly, or after certain stages in cyclical metamor- 

 phosis, they could reproduce. For cyclical self-repetition in the 

 way of parentage, being eminently the characteristic of living 

 organisms, as opposed to non-living objects, no classification of 

 such organisms would be either natural or valuable which did not 

 lay that particular part of their history in a compact and manage- 

 able form before the mind, whensoever evidence as to it was ob- 

 tainable. It is true that such evidence is by no means invariably 

 accessible ; and when it is not, we have only likeness to guide us 

 as to saying that it is more or less probable that between any two 

 organisms such prospective and retrospective community in parent- 

 age might or did exist. On the other hand, where this evidence is 

 forthcoming, the question of identity of species is instinctively 

 and at once settled in the affirmative ; even when the unlikeness 

 between the individuals compared may be as extreme as that 

 which exists between the well-known larvae of Batrachia and their 

 adult forms, or between the less familiar but even more strikingly 

 differing larvae and adults of Cirripedia, of other Crustacea, of 

 the Platyelminthes, and of many Hydrozoa (see infra, pp. 162, 

 345, 252). The theory of evolution with which Mr. Darwin's 

 name is connected, asks us to deal with species in their relation 



of trees ; as systems of mountain-ranges, more or less connected by outliers ; or, 

 happiest metaphor of all— as the islands of an archipelago, sometimes all but conti- 

 nuous through the intermediation of connecting reefs, sometimes sharply separated 

 by unfathomable seas. 



