ZOOLOGY. 333 



absolutely destructive. Now, geological epochs, as at present under- 

 stood, are defined by peculiar assemblages of species, and the amount 

 of change in the organic contents of proximate formations or strata 

 is usually accepted as a measure of the extent of the disturbances 

 that affect them. Yet this latter inference, involving as it does the 

 supposition that the spread and continuity of species in time is 

 dependent upon physical influences, is adverse to the notion of a 

 life of a species as 'stated above. If we seek for the origin of this 

 notion we shall find that it has two sources, the one direct, the other 

 indirect. It is not an induction, nor pretended to be, but an hypo- 

 thesis assumed through apparent analogies. Its first and principal 

 source may be discovered in the comparison suggested by certain 

 necessary phases in the duration of the species with others in the 

 life of an individual, such as each has its commencement, and each 

 has its cessation. Geological research has made known to us that 

 prior to certain points in time, certain species did not exist, and 

 that after certain points in time, certain species ceased to be. The 

 commencement of a species has been compared with birth, the 

 extinction with death. Again, many species can be shown to have 

 had an epoch of maximum development in time. This has been 

 compared with the maturity of the individual. Between the birth of 

 an individual and the commencement of a species in the first appear- 

 ance of its protoplast, the analogy is more apparent than real. We 

 know how the former phenomenon takes place, but we have no 

 knowledge of the latter. Between the maturity of the individual 

 and the maximum development of a species there is no true analogy, 

 since the latter can easily be proved to be entirely dependent on the 

 combination of favoring conditions, and during the period of duration 

 of a species, there may be two or more epochs of great or even equal 

 development, and two or more epochs of decline alternating with 

 epochs of prosperity. The epoch of maximum of a species may also 

 occur during any period in its history short of the first stage. Geo- 

 logical and geographical research equally show that the flourishing of 

 a species is invariably coincident with the presence of favoring, and 

 its decline with that of unfavorable conditions. Hence, there is no 

 analogy between the single and definite phase of maturity of the 

 individual, and the variable and sometimes often-repeated epochs of 

 luxuriant development in the duration of a species. Between the 

 death of the individual and the extinction of a species, there is an 

 analogy only when the former event occurs prematurely through the 

 influence of destroying conditions. But in their absence, an indi- 

 vidual, after its period of vitality has been completed, must neces- 

 sarily die ; whereas, we have no right to assume that such would be 

 the fate of a species so circumstanced, since in every case where we 

 can either geologically or geographically trace a species to its local or 

 general extinction, we can connect the fact of its disappearance with 

 the evidences of physical changes. The second and more indirect 

 source of the notion of the life of a species may be traced in apparent 

 analogies, half-perceived, between the centralization of generic groups 



