1346 The Zoologist — September, 1868. 



through two generations in a year, and for the two generations to differ 

 entirely in coloration and size : in every instance where this is the 

 case, entomologists have treated the vernal and autumnal generations 

 as species : it is scarcely needful to say that they are not so. 



It is well known that the duration of individual life varies in 

 different species; thus some insects pass through four or five genera- 

 tions in a year, although the usual term of life is twelve months : 

 rabbits live four or five years, cats six or eight, dogs ten or twelve, 

 horses sixteen or eighteen, and ravens and tortoises are said to live for 

 centuries. This diversity in the term of years allotted to individual life 

 has no ascertained relation to that allotted to the life of the species of 

 which that individual forms an integral part : on this subject there are 

 no statistics : the death of a species is an unobtrusive event, while that 

 of an individual, whether an emperor or a fly, is an event that frequently 

 forces itself on our notice : so that neither the brevity nor prolongation 

 of individual life affords us any clew to the duration of specific life. 



How then do we acquire a knowledge of the death of species ? 

 first, from the testimony of the rocks ; secondly, from beholding with 

 our own eyes the partial death of species ; in other words, their extinc- 

 tion in certain limited localities; thirdly, from historic evidence. 



Rocks contain the monuments of countless thousands of species: 

 our lists of fossils (so called) are obituaries of species ; but our know- 

 ledge of them being imperfect, they serve rather to excite than to 

 satisfy our curiosity respecting the inhabitants of that vast cemetery 

 of species, the superficial covering or crust of the earth. History has 

 erected tombstones over a few ; and imagination, hypothesis, theory, — 

 call it what you will, — is even now preparing other tombstones not yet 

 required, carving prospective epitaphs for species about to die. A 

 great deal is said and written against the prevalent practice of pro- 

 mulgating prophetic dicta in physical science, but certain coming 

 events cast such unmistakable shadows, or, more properly speaking, 

 project such a blaze of light before them, that the general character of 

 the future stands revealed. Those who have studied earth's in- 

 habitants, with or without the assistance of Geology, have acquired a 

 very fair insight into the future, and are gradually led to perceive, 

 accept, and even enunciate results; yes, to enunciate them with a 

 confidence which the dissentients call " dogmatism," the cavillers 

 "hypothesis," the ingeuuous "theory," and the approvers "logical 

 deduction." Taking refuge under this diversity of sentiment, and 

 certain that no expression of opinion can jeopardise the truth, I venture 



