THE EVIDENCE FROM DEGENERATION. 359 



enough explain, not merely why the sense organs arise from the skin 

 surface, but also why the brain grows outwards 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 

 retrogression 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 

 degeneration 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 un- 

 likely kind ; and amongst these possibilities, that of extreme degene- 

 ration 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 of different 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 nature 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 arrangements. 

 Plants, on the other hand, are specially constructive and elaborative 

 in their feeding. They build up from the non-living matters 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 autumn's fruits, re- 

 presents, 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 re-echoes what commonplace 

 observation daily demonstrates. Hence, it may be a natural 

 enough inference that whatever 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 retrogression and degenera- 

 tion of the animal type. Many animals are thus known to develop 

 chlorophyll^ or the green colour we see characteristically in every 



