I have not named my authorities or given references to any passages in their books. My critics, friendly or unfriendly, may complain of this omission. But I hope they will not. I hope they will see that I have gathered my materials together for a clearly shown purpose with which particular references and frequent defined quotations would have interfered. I wanted to build a wayside cottage for travellers who are in haste and will soon pass on, not a museum for the leisurely student. I hope, then, that my critics will criticize the cottage to their hearts' content-I shall do my best to learn from them, -but I beg them not to treat it as a museum whose curator has either not had the sense or not taken the trouble to ticket its contents. In place of references I have given in an appendix two short lists of easily accessible books which will give technical support to the substance of this little work. I have learnt much from them and taken-all but quoted-much. I make no apology for including, among books from which I have learnt, two of my own. Nothing teaches as solidly as trying to teach. And the record of a learner sometimes helps learners, when the oracles of the learned fail. But I must acknowledge here my own great indebtedness to two of those learned, my old instructors, Sir Edward Schäfer and Sir Edwin Ray Lankester, whose admirable lessons in the biological sciences of which they are distinguished professors laid a scientific foundation in me for all my subsequent study.