388 THE POPULAR SCIENCE MONTHLY. 



eyes falls to be considered, the affinity of the skin-layer and the ner- 

 vous system is a fact worth noting. It is this truest of relationships 

 which may reasonably enough explain, not merely why the sense-organs 

 arise from the skin-surface, but also why the brain grows outward to 

 meet with the structure to which it is so near akin. 



Degeneration of a very pronounced kind thus accounts for the 

 peculiarities of sea-squirt structure to-day. The case of ascidian re- 

 trogression is likewise the more interesting, seeing that its reverse 

 side is that of progressive evolution and development of the highest 

 forms of life the existing world knows. It is, therefore, important to 

 note in passing that the possibilities of development may include de- 

 generation of a very marked type, along with progressive evolution of 

 equally pronounced kind. The category of life's extension includes, 

 in fact, many possibilities which at first sight might appear of most 

 unlikely kind ; and, among these possibilities, that of extreme degen- 

 eration is by no means the least notable as an element in inducing the 

 material variety of life we behold in the animal and plant worlds of 

 to-day. The list of causes which lead to the degeneration of living 

 beings includes, however, other fashions of producing retrogression 

 than by fixation and parasitic habits, and operates in different ways 

 upon organisms of varied structure and social or biological rank. 

 Changes in food and feeding may thus accomplish degeneration and 

 induce physiological backsliding of the most typical description. It 

 is a familiar fact that the animal organism is of relatively higher nat- 

 ure than the plant, seeing that the animal frame can, as a rule, feed 

 upon and build up its tissues from organic or living matter only. 

 Animals, in other words, demand the substance of other animals or of 

 plants, or of both combined, as a necessity of their commissariat ar- 

 rangements. Plants, on the other hand, are specially constructive and 

 elaborative in their feeding. They build up from the non-living mat- 

 ters around them carbonic acid, water, ammonia, and minerals the 

 tissues of their living bodies. They "transubstantiate" this non- 

 living matter into living tissue ; and the verdant tints of spring, the 

 full glory of the summer's blossom, or the mellow ruddiness of au- 

 tumn's fruits, represents, each in its way, the result at once of the 

 plant's constructive chemistry and of the elaboration into living matter 

 of the inorganic materials of air and soil around. 



The animal frame, therefore, presents us amid exceptions to the 

 above rule in both animal and plant series with relatively greater 

 complexity of organs and tissues than the plant-body presents. This 

 statement simply reechoes what commonplace observation daily de- 

 monstrates. Hence, it may be a natural enough inference that what- 

 ever causes tend to bring the animal feeding nearer in type to that of 

 the plant will tend to simplify animal structure, and so to produce re- 

 trogression and degeneration of the animal type. Many animals are 

 thus known to develop chlorophyl, or the green color we see charac- 



