306 THE EVOLUTION OF ANIMAL BEHAVIOUR 
in the case of many organic species, in all those, for ex- 
ample, which we classify as plants, must not be taken as pre- 
sumptive evidence that in other species, for instance in the 
multitudinous host of insects, the development of conscious 
situations is of no biological value. The fact that chlorophyll 
is not developed in any mammal does not show that the 
possession of this substance is of no service to the higher plants. 
It would not be worth while to give expression to this very 
obvious truth, were it not that critics of natural selection 
persistently argue that because one species gets on perfectly 
well without this or that particular character it can have 
played no part in securing the survival of another species. 
When I described, at a meeting of naturalists, how well young 
chicks could swim, such a critic drew me aside after the 
meeting, and expressed his surprise that this did not convince 
me that the webbed foot of the duck could not logically 
be attributed to natural selection. This is an extreme case, 
and one obviously taken on peculiarly weak grounds. But 
even Huxley urged that, because a frog, from which the 
cerebral hemispheres have been removed, performs many 
co-ordinated actions without conscious guidance, consciousness 
is, throughout nature, merely an accompaniment of certain 
molecular changes in the brain. “Such a frog,” he says,* 
‘“‘walks, hops, swims, and goes through his gymnastic per- 
formances quite as well without consciousness, and consequently 
without volition, as with it; and if a frog, in his natural 
state, possesses anything corresponding with what we call 
volition, there is no reason to think that it is anything but 
a concomitant of the molecular changes in the brain which 
form part of the series involved in the production of motion. 
“The consciousness of brutes,” he continues, ‘ would 
appear to be related to the mechanism of their body simply 
as a collateral product of its working, and to be as completely 
without any power of modifying that working as the steam- 
whistle which accompanies the work of a locomotive engine is 
without influence on its machinery. Their volition, if they 
* “Collected Essays,” vol. i. p. 249. 
