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194 NOTICES OF THE MEETINGS [May 7, 
destructive. Now, geological epochs, as at present understood, 
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 sup- 
position that the spread and continuity of species in time is depen- 
dent 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 hypothesis 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 develop- 
ment 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 appearance 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 de- 
velopment of a species there is no true analogy, since the latter 
can easily be proved to be entirely dependent on the combination of 
favouring conditions, and during the period of duration of a species 
there may be two or more epochs of great or even equal develop- 
ment, 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. Geological 
and geographical research equally show that the flourishing of a 
species is invariably coincident with the presence of favouring and 
its decline with that of unfavourable 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 individual after its period of vitality has been 
completed must necessarily 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 geogra- 
phically trace a species to its local or general extinction, we can 
