As every fresh branch of investigation in natural history has a tendency to gather around it a rapidly accumulating literature, some explanation may probably be looked for from an author who offers a new contribution to the public. And when, as in the present instance, the writer's intentions are of an humble kind, it is the more desirable that he should state his views at the outset. Nor can the force of this claim be supposed to be lessened, from the gratifying fact, that the present writer has already received a warm welcome from the public.