iv 
INTEODUCTTON. 
This assumption, if persistently followed out, would of course account for the greatest 
diversities that exist in Nature, if sufficient time were allowed ; but it is obvious that, however 
far it may account for multiplication and modification of species, it cannot account for ulti- 
mate origin ; for it ^resupjposes life : this puts one limit on its action, and the discoveries in 
geology, though the most diverse opinions have been held respecting their real force, seem 
to require a still further limitation. The laws of generation apply equally to the vegetable 
kingdom ; and here too we find the universal individual differences ; also the close approxi- 
mation of forms, and apparent gradation from group to group, which characterizes the internal 
arrangement of each kingdom, is equally perceptible on the line of junction; and the pro- 
gression of form from vegetable to animal is as complete as from fish to reptile, or from bird 
to mammal. If these gradations are held to be good evidence of a common origin in the 
one case, they must be equally so in the other ; and the animal world, with the vegetable, 
must be referred to a single stock ; and on the other hand, if they can be shown to be com- 
patible with independent origin in the one case, we may assume that they are so also in the 
other. From geology, we learn that both animal and vegetable life appeared in the oldest 
fossiliferous stratum, and that, though the animal forms came later and in less variety than 
the vegetable, the intermediate stages have neither been discovered, nor has there been any 
break in the continuity of the stratum in which modification may be assumed to have taken 
place unrecorded ; and further, as the conditions were evidently favourable for preserving 
their remains had they existed, the inference may fairly be drawn that they did not or, in 
other words, that the origins of the two kingdoms were independent. If this be the case, 
the minute gradations of form still existing between animals and vegetables is shown to be 
no proof of common origin ; and we may go one step further, and assume that it may also be 
no proof in the case of the various orders, families, and genera of the animal kingdom. 
If we follow out this idea through the succeeding fossil-bearing strata, we find, speaking 
in general terms, that the great divisions of the animal kingdom appear on the stage in suc- 
cession, and that each succeeding era is more or less distinctly characterized by the advent of 
a new and higher order coexisting with the preceding forms, but with no evidence of minute 
gradations of forms connecting them. In most instances these new eras are preceded by 
blank intervals of unknown duration ; and in them the intermediate forms might have been 
assumed to have existed and been destroyed, had we not the evidence of the period of the 
first introduction of animal life to guide us in assuming the contrary. 
Again, we have the fact that throughout the whole period of deposition of fossils up to the 
present day the animal and vegetable kingdoms have existed side by side, and the gradations 
of form between them, though fine, are imperfect, and were no finer or more gradual in the 
Upper Silurian period than they are at the present day. These various results would seem 
to indicate that the law of generation does not entirely fulfil the conditions of the problem, 
and that there are other forces controlling and counteracting this multiplication of individual 
differences, and that the law, instead of being a single force, is rather the resultant of a 
number of forces which, compensating each other, produce the harmonious whole that we 
see around us. The minute gradations of species points to certain amount of affinity ; the 
