thieves, because his superior strength cannot be brought into action against his aerial foes, but the Diplodocus would be as helpless as a sheep against his carnivorous rapacity. The great Irish elk is said to have become extinct in consequence of the excessive development of the antlers, which ultimately became so heavy, that when the animal stumbled into a bog he could not extricate himself.
Correspondence between the Senses and Environment.
There is one other line of thought to which I should like to direct attention, and that is, the correspondence which exists between the senses and certain phenomena in the environment. Animals, as you are aware, keep up their connection with the external world by means of five senses, which are not inappropriately designated the five gateways of knowledge. The mechanism through which these different senses perform their function, consists of more or less elaborately constructed organs the degree of elaboration being generally proportionate to the position of the species in the scale of development ranging from the merest grouping of a few nerve-cells up to the complex structure of the brain and nervous system of the higher vertebrates. Now, what I wish to point out more particularly is, that these different senses have extremely well-defined counterparts in the environment, such as light, atmospheric undulations, and certain physical and chemical properties of matter, so that each sensory apparatus has a special excitant in the external world to which it responds. The natural phenomena to which the senses are thus so remarkably correlated may be regarded as constant quantities in nature, and hence they produce cumulative effects on living organisms susceptible to improvement or advance in life. The subject presents an ideal field for the action of natural selection as defined by Mr Darwin. Among the lower animals there is great irregularity in the degree of development of the senses, some having no localised organs, and others only rudimentary ones. The majority of molluscs are endowed with the sense of smell, and some land-shells are guided to their food by taste as well. The cephalopods and gastropods are furnished with visual organs, while most of the bivalves
