16 SCIENCE PROGRESS. 
tained to be characteristic of the vegetable kingdom as well 
as the animal. Thus the species of a family or genus are 
often distributed round a centre ‘‘ which seems to be the 
principal focus or favourite seat of the tribe,” from which 
the branches diverge in various directions. The particular 
species, when compared, can be referred to ‘one type of 
organization”. The slighter differences between them 
“seem to lose themselves in the sameness of form be- 
longing to the genus, and even suggest a suspicion that 
they all proceeded from one original. The phznomena 
of resemblance must have had their sufficient reason as 
well as those of diversity.” He then inquires whether the 
explanation is to be found in the action of ‘some slight 
modification in the productive causes” which stamped the 
genus at its first appearance “ with all these specific diversi- 
ties”; or whether on the other hand a uniform genus was 
first created which ‘afterwards became diversified by the 
imiduence of external) agents .) Ele coneludes) that (the 
former of these alternatives is more strongly indicated by 
the knowledge of his time. 
‘Whichever of these suppositions may be true in point 
of fact, the separation of families and genera into particular 
species, and the distribution of these species to particular 
habitations, according to their physical properties, is evi- 
dently a part of the provision of nature for replenishing 
the earth with organized inhabitants, placed everywhere 
according to the congruity of soils and temperatures, 
with their structure and habitudes. 
“ But why is it to be supposed that the influence of this 
law of adaptation has stopped here? Is it not probable 
that the varieties which spring up within the limits of 
particular species, are further adaptations of structure to 
the circumstances under which the tribe is destined to exist ? 
Varieties branch out from the common form of a species, 
just as the forms of species deviate from the common type 
of a genus. Why should the one class of phenomena be 
without end or utility, a mere effect of contingency or 
chance, more than the other ? 
“There are indeed many instances in which we can 
