84 THE POPULAR SCIENCE MONTHLY. 



out in many directions, but the possible height attainable by each gen- 

 eral 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 Echino- 

 derms, 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 mo- 

 tion 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 in- 

 ternal 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 dis- 

 advantages, 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 sen- 

 sory acuteness. The armored animals are necessarily heavier, less ac- 

 tive, 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 spe- 

 cialized ; 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. 



