18 ZOOLOGICAL EXERCISES. 
logy has taught us that there has been a constant succession 
of life on the earth, without a break, and that the lower 
organisms have preceded the higher; that we cannot find 
transitional forms between most of the species, and that 
whole groups of animals seem to have appeared suddenly is 
no doubt due to the very imperfect state of the geological 
record. In many cases we have perfect transitions between 
living species, making it impossible to assign limits to 
each. All these facts are, no doubt, compatible with the 
idea that each species has been separately created ; but it is 
highly improbable that species should have been specially 
created on the same plan as would naturally have resulted 
from descent with modification. There are also many 
classes of facts which are inconsistent with the special 
creation theory. For example, species of the same genus 
often vary in the same way, and it appears almost certain 
that these variations must be due to inheritance. Abnor- 
mally developed organs vary more than normally developed 
ones, owing, doubtless, to their having been lately de- 
veloped; this would not be the case by the special creation 
theory. Many animals possess rudimentary and useless 
organs which they ought not to have, on the supposition of 
special creation, but which are easily explained by the 
theory of descent. Islands generally have a considerable 
number of species that are found nowhere else, but these 
species are always related to the animals on the mainland, 
and they belong to groups, such as birds, which possess 
considerable powers of locomotion ; it is certainly absurd to 
suppose that, on islands, only those species should have been 
created which belong to groups possessing sufficient powers 
of locomotion to get to the islands. 
The question is thus summed up by Professor Sachs :— 
“The theory of descent involves only one hypothesis that is 
