190 
DIFFICULTIES ON THEOEY. 
Chap. VI. 
passed, we should have to look to very ancient ancestral 
forms, long since become extinct. 
We should be extremely cautious in concluding that 
an organ could not have been formed by transitional 
gradations of some kind. Numerous cases could be 
given amongst the lower animals of the same organ 
performing at the same time wholly distinct functions ; 
thus the ahmentary canal respires, digests, and excretes 
in the larva of the dragon-fly and in the fish Cobites. 
In the Hydra, the animal may be turned inside out, 
and the exterior surface will then digest and the sto¬ 
mach respire. In such cases natural selection might 
easily specialise, if any advantage were thus gained, a 
part or organ, which had performed two functions, for 
one function alone, and thus wholly change its nature 
by insensible steps. Two distinct organs sometimes 
perform simultaneously the same function in the same 
individual; to give one instance, there are fish with 
gills or branchim that breathe the air dissolved in the 
water, at the same time that they breathe free air in 
their swimbladders, this latter organ having a ductus 
pneumaticus for its supply, and being divided by highly 
vascular partitions. In these cases one of the two or¬ 
gans might with ease be modified and perfected so as to 
perform all the work by itself, being aided during the 
process of modification by the other organ; and then 
this other organ might be modified for some other and 
quite distinct purpose, or be quite obliterated. 
The illustration of the swimbladder in fishes is a good 
one, because it shows us clearly the highly important 
fact that an organ originally constructed for one pur¬ 
pose, namely flotation, may be converted into one for a 
wholly different purpose, namely respiration. The swim- 
bladder has, also, been worked in as an accessory to the 
auditory organs of certain fish, or, for I do not know 
