THE EVOLUTION OF ORGANIC FORM. 91 



These swimming bells are not unlike the Medusse in this particular, 

 but have become far more specialized than the Medusse, as they possess 

 none of the organs requisite to individual life. 



In the Physalia, or Portuguese man-of-war, the connecting body 

 is developed into a floating bladder, moving by force of the winds, and 

 with its variously modified polyps beneath it. 



Such are some of the modes adopted by Nature to produce free mo- 

 tion in the lower types of animal life. The animals produced by this 

 social subordination of function are imperfect because the subordina- 

 tion is indefinite. There is not a single organ adapted to each func- 

 tion, but a variable number. And the very means by which propulsion 

 through the water is gained renders this imperfection necessary. For, 

 if a single individual constituted each organ, the animal would become 

 compact, and be moved by a single contracting bell. Its powers of 

 motion would be reduced to those of the Medusae, and its organization 

 retrograde toward the original compact stage. 



This line of progress, with its necessarily imperfect specialization, 

 is evidently incapable of attaining the level of the Echinoderm, much 

 less of the mollusk. 



But another line that of the segmented animals seems much 

 better adapted to attain a high grade of evolution. Not but that its 

 segments possess anatomical characters as stubborn as those of the 

 Radiates, but that these are less restrictive to a high evolution. 



It is, of course, not the usual view to consider the Articulates as 

 the result of an original social organization. The segments, in the 

 higher genera, are so specialized that they now exist but as organic 

 parts of a single animal. And yet, if we consider the lower articulated 

 worms, evidences of such an origin may be discovered. 



In these lowest Articulates scarcely any difference is to be traced 

 between the segments. The anterior, from its position, acts as a 

 mouth, but otherwise they are as similar as the individual Salpse. But 

 the most significant feature is that in many cases each of them possesses 

 the organization of an individual. Each segment still retains its separate 

 nervous ganglion, its separate muscles, its separate limbs, frequently 

 its separate breathing organs, and, in a partial degree, its separate cir- 

 culation. These are only subordinated to the extent of being joined 

 by connecting links, while the intestine of each becomes continuous as 

 a common intestine. 



In fact, this organic individuality is carried, in certain cases, to a 

 yet more significant extent. The organs of sj^ecial sense the most 

 highly specialized of animal organs are, in some instances, retained 

 by the separate segments. There are not only existing worms with 

 eyes at each extremity of the body, but others which possess eyes in 

 each separate segment. 



Thus we are led not alone to the conception of an original animal 

 which became associated into the Articulates, but even to some idea 



