362 CHAPTERS ON EVOLUTION. 



of living histories become theoretically plain ; and it argues hopefully 

 for the correctness and value of the doctrine before us that it has, so 

 far as it has been logically pursued, fitted compactly and harmo- 

 niously enough with ascertained facts and with received views of the 

 origin of animals and plants. That higher forms of life than the sea- 

 squirt and insect race are by no means exempt from the influence ot 

 retrogressive change, is an observation worth noting at the close of 

 our researches. We know, for instance, of lowly structures in shell- 

 fish life appearing in the midst of highly organised frames. A 

 mussel, a cockle, or an oyster, whose early development, as we have 

 seen, runs in parallel lines to that of the snail and whelk-class, is 

 nevertheless esteemed less highly organised than the latter. The 

 mussel or oyster-tribe possess no head. The snails and their allies, 

 as every one knows, not merely exhibit a well-developed head, but 

 have that extremity provided with eyes, tentacles or feelers, and 

 other addenda of the front region of the animal body. Hence it is 

 more than probable that the mussel, headless and enclosed in its 

 shell, and possessing relatively little interest in the affairs of the 

 outer world, is an example of a degenerate type of molluscs. The 

 mussels and their relations stand, in fact, at the opposite extreme ot 

 development in this respect from those well-known molluscs the 

 cuttlefishes. In these creatures, the tendency to head-development 

 or what Professor Dana calls "cephalisation" reaches its maximum, 

 as any one may readily enough suppose on looking at an octopus or 

 squid, with its great head, its enormous eyes, and its nerves massed 

 together to form a brain enclosed in a kind of skull. Even as com- 

 pared with the earlier cuttlefishes whose shells, under the name 

 of ammonites and the like, we find fossilised in large numbers 

 the squids and cuttles of to-day present, in the extreme development 

 of head, a noteworthy advance. 



Thus, whilst the one Molluscan tribe of mussels and their 

 neighbours has degenerated and gone to its own lowly place 

 in the series, other groups starting on an equal footing have 

 advanced, and, through progressive evolution, have produced 

 those higher manifestations of molluscan life that teem in the 

 seas of to-day. Even amongst the Vertebrate animals we meet 

 with examples of degenerative tendency which are not so easily 

 explicable as the foregoing illustrations. In most snakes only one 

 lung is fully developed as a rule, the companion organ being rudi- 

 mentary and degenerate. In birds, the egg-producing organs are 

 similarly developed on one side only. How degeneration should 

 be thus partial, and affect one-half of an animal's frame, so to speak, 

 is very hard to discover. External conditions of life and the in- 

 fluences of surroundings could apparently possess little effect in 

 inducing such an unsymmetrical retrogression of parts. Most 



