EVOLUTION OF THE HUMAN FORM. 505 



but various types of animals became defensive rather than offensive in 

 habit. These include the armored classes, of vs^hich the mollusks are 

 the most marked example. To these may be added the forms that 

 seek concealment, either by burrowing or otherwise. These creatures 

 are necessarily sluggish, either from the weight of their armor or their 

 lurking habits. They live upon inactive food, their environment is 

 limited, their contact with nature narrow, their powers of sensation 

 and consciousness little developed. The conditions of their life defi- 

 nitely take them out of the line of the higher progress, in which they 

 can not compete with the more active forms. 



In considering then the classes of animals adapted to advanced 

 development, it seems necessary to confine ourselves to the free-mov- 

 ing, agile forms. And among the inhabitants of the ocean — in which 

 life had its origin and its lower stages of development — these are not 

 to be sought among the crawling and burrowing, but among the swim- 

 ming species. With these the highest activity is dependent upon the 

 most suitable formation of body and the most capable organs of motion. 



If we may pursue our fable of nature's experiment in evolution, 

 it can be said that very numerous trials in form were made. There 

 seem possible to colloid substance only two general types of form, the 

 circular or radial and the elongated. Both these were produced in 

 numerous varieties, the circular type embracing two large classes of 

 animals, the coelenterata and the echinodermata, all of them sluggish, 

 many of them sessile, their general shape and radiated limbs being 

 very ill adapted to active motion. In this respect they were at a great 

 disadvantage as compared with the bilateral, elongated type. 



We thus seem to find the experiment of organic evolution, after 

 millions of years of incessant effort, reaching the type which in its 

 simpler stages is popularly designated as the worm, as the form best 

 adapted for advanced evolution. The pristine worm was not in itself 

 a promising creature. Its organs of motion were inefficient and its 

 movements sluggish. Probably several worm-like types appeared, 

 simply organized elongated animals of varied formation, to which we 

 owe, in their final development, the three classes of animals known 

 as the moUusca, the arthropoda, and the vertebrata. This develop- 

 ment of an elongated, bilateral animal would seem to have been an 

 inevitable stage in the evolution of animal life, sure to appear in any 

 planet where life had sufficiently progressed, and capable of unfold- 

 ing into a number of different types. In addition to the great types 

 named, several of minor importance appeared upon the earth, and 

 different ones may well have arisen elsewhere. 



Yet if we seek for the highest class of form likely to arise from the 

 worm-like unit, our field of search is restricted. If activity and flexibility 

 of body are advantageous, we must seek these in the swimming rather 



