ADDRESS. xci 
It is a part of sound knowledge to be able to recognize the subjects re- 
garding which we have not, at present, the basis of true assertion. 
On the few occasions in which I have been led to offer observations on 
the probable cause of the extinction of species, the chief weight has been 
given to those gradual changes in the conditions of a country affecting the 
due supply in sustenance to animals in a state of nature, I have also pointed 
out the characters in the animals themselves calculated to render them most 
obnoxious to such extirpating influences; and on one occasion* I have ap- 
plied the remarks to the explanation of so many of the larger species of par- 
ticular groups of animals having become extinct, whilst smaller species of 
equal antiquity have remained. 
In proportion to its bulk is the dificulty of the contest which, as a living 
organized whole, the individual of such species has to maintain against the 
surrounding agencies that are ever tending to dissolve the vital bond and 
subjugate the living matter to the ordindry chemical and physical forces. 
Any changes, therefore, in such external agencies as a species may have 
been originally adapted to exist in will militate against that existence in a 
degree proportionate, perhaps in a geometrical ratio, to the bulk of the 
species. If a dry season be gradually prolonged, the large mammal will 
suffer from the drought sooner than the small one; if such alteration of 
climate affect the quantity of vegetable food, the bulky Herbivore will first 
feel the effects of stinted nourishment; if new enemies are introduced, the 
large and conspicuous quadruped or bird will fall a prey, whilst the smaller 
species conceal themselves and escape. Smaller animals are usually, also, 
more prolific than larger ones. 
“‘ The actual presence, therefore, of small species of animals in countries 
where larger species of the same natural families formerly exisied, is not the 
consequence of any gradual diminution of the size of such species, but is the 
result of circumstances, which may be illustrated by the fable of the ‘ Oak 
and the Reed ;’ the smaller and feebler animals have bent and accommodated 
themselves to changes which have destroyed the larger species.” 
Accepting this explanation of the extirpation of species as true, Mr. 
Wallace+ has recently applied it to the extirpation of varieties; and, assu- 
ming, as is probable, that varieties do arise in a wild species, he shows how 
such deviations from type may either tend to the destruction of a variety, or 
to adapt a variety to some changes in surrounding conditions, under which 
it is better calculated to exist, than the type-form from which it deviated. 
No doubt the type-form of any species is that which is best adapted to 
the conditions under which such species at the time exists; and as long as 
those conditions remain unchanged, so long will the type remain; all 
varieties departing therefrom being in the same ratio less adapted to the 
environing conditions of existence. But, if those conditions change, then 
* On the Genus Dinornis (part iv.), Zool. Trans. vol. iv. p. 15 (February 1859), 
T Proceedings of the Linnean Society, August 1858, p. 57. 
