out in many directions, but the possible height attainable by each general direction of growth is limited by certain principles, which we may be able to discover.
Both herbivorous and carnivorous animals may exist in fixed and in motile forms—food-attracting and food-seeking adaptations. The fixed forms are principally or entirely water-animals, comprising the Sponges, a large section of the Polyps, the lower forms of the Echinoderms, with some divergent forms, such as the Bryozoa, the Tunicata, and the Barnacles.
These are saved the necessity of moving, by the fact of their being tenants of a liquid whose moving currents bring them food, and by being capable of themselves producing water-currents, on which food is borne to them. Their necessary movements are reduced to the motion of tentacles—current-making or food-seizing organs. No sense is requisite except touch, and therefore no higher degree of sensibility is developed. These fixed forms thus necessarily remain at the foot of the ladder of progress, being but a step above the Protozoa, or single celled animals. They may be classed, however, as superior to the internal parasites of animals, which live by imbibing elaborated animal juices, and need no motile nor sense organs.
But, as soon as an animal obtains powers of free motion, it comes at once into contact with a much wider range of conditions and needs to gain extended powers. It is, moreover, placed under seeming disadvantages, which are really of high efficacy in its development. It possesses no stone castle of refuge, from which it has but to extend its retractile arms. It is, therefore, exposed to much greater dangers, its whole body being open to the assault of foes.
There are two general methods by which protection from these perils is gained: the first by armor; the second by activity and sensory acuteness. The armored animals are necessarily heavier, less active, and less flexible, than the unarmored. The latter depend for safety on activity and variety of motion, on quickness of sense, and on weapons of defense. They are, consequently, more highly developed than the armored, whose firm coating forms their main protective adaptation.
They also come into contact with a much wider range of natural conditions, their more extensive excursions accustoming them to more varied forms of food, adapting them to wider surface and temperature relations, and exposing them to more numerous foes. Thus they must become fitted to a wider environment, and their powers be more specialized; the naked, flexible, active animal being thus necessarily the highest in point of development.
These general views lead us to their particular application to the existing animal types. We think it can be shown that each type has had full opportunities of unfoldment, and has reached the extreme limit of its line of growth.