C.—GEOLOGY. 61 
observation that the geological record is one of extinction and replacement 
in so far as species and even higher taxonomic divisions are concerned. 
The great weight of geological evidence points to the supplanting of 
one species by another, not to the transformation of species into their 
successors. A single transformation sequence may be regarded as suflicient 
to establish the principle, but an adequate explanation must be given of 
_ the failure of vertical seriations in the great majority of cases. This 
explanation is not yet forthcoming, and its lack stands as the chief item 
in the contra account of the balance sheet of evolution. 
Le Conte explained this generally observed replacement as due to 
the abruptness of the change in environment. The hereditary tendencies 
maintained to the breaking point and suddenly gave way. The result 
was a great mortality and the survival of only a few individuals showing 
transitional stages. Unfortunately for this explanation, the advent of a 
new species is generally unheralded by even a few individuals showing 
the connexion with an earlier species. 
Migration is the generally accepted explanation of the abruptness of 
faunal changes, but this is a mere statement of the evident, and throws 
no light on the evolution of the immigrant species. 
Let us assume a species to be in possession of a given area either of 
land or of water. This area of similar conditions, in most cases, can have 
no exact boundaries. It seems safe to infer that the individuals towards 
the centre of the region are more closely adapted than those on the margin, 
although no apparent anatomical differences exist. It would follow that 
on the advent of changed conditions the individuals approaching close 
adaptation, 7.e. those towards the centre of the region, would all succumb, 
but that those with a wider latitude of adaptation inhabiting the borders 
of the area would in part survive. 
The fate of the survivors would depend on the nature of the change, 
which might be for better or for worse. Favourable changes, from the 
point of view of the animal, could be only those which lessen the necessity 
for adaptation, which bring it nearer to complete adaptation, but which 
render it less able to resist further changes of an unfavourable kind. For 
example, a species lives in an area with a constant temperature of 60° 
at the centre, but varying from 50° to 60° at the margin. If the tempera- 
ture falls to 55° throughout, all the animals at the centre will die, but those 
at the borders will survive and will find themselves in better circum- 
stances than before the change. Minor evolution only can result. 
_ It is to the unfavourable changes, therefore, that we must look for 
an explanation of the more deeply seated organic evolution ; by unfavour- 
ble meaning adverse to the present condition of the animal in that it is 
forced to further adaptation. A change of this kind is not of necessity 
adverse to life; it may even be stimulating. The animals towards the 
“margins of a colony, by reason of their less perfect adaptation, may in a 
few instances survive an unfavourable change. The first impulse of these 
‘survivors will be to escape by flight, and thereby diminish the fatal 
suddenness Of the change and thus achieve adaptation. 
_ The new species would arise rather suddenly with but few individuals 
of the transition stages. Arrived at a favourable habitat, migration 
tvould cease, multiplication would ensue, and closer and closer adaptation 
would be achieved, Eventually an approximation to perfect adaptation 
