234 CHAPTERS ON EVOLUTION. 



simply a later modification of the worm-stock. The life history of a 

 sea-butterfly or pteropod, in fact, takes up the narrative where the 

 development of chiton ends it. Chiton led us to the worm-larva 

 stage, and thereafter branches off on its lower molluscan path. But 

 the pteropod may, as we have seen, begin life as the worm (Fig. 153), 

 and proceeds not merely to develop its " veliger-stage," but remains 

 permanently therein ; flapping its way over the surface of the sea by 

 means of the permanent " velum," or its substitute, in the form of 

 the fins or wings. Last of all, a gasteropod like ALolis presents us 

 with a multum in parvo of the whole process of gasteropod and 

 molluscan evolution. Here, we take up the story at the stage where 

 the pteropod history concluded. ^Eolis and its neighbours (or 

 Lymneus), passing through the pteropod stage, each with its "velum," 

 develop onwards to become the higher and shelled gasteropod, and 

 represent the furthest evolution of the race. Thus, from the worms 

 to the lampshells on one hand, and to the chitons and " Toothshells " 

 on the other ; from these latter, in turn, to the pteropod and thence 

 to the bivalves and gasteropods, the track of development seems 

 plainly marked. The whole story it tells is that of a shifting panorama 

 of the modification of the animal form ; phase succeeding phase, and 

 each new succession of forms obscuring, or it may be intensifying, the 

 development of the preceding classes and groups. But, clearly marked 

 or obscure, understood fully or only suggested to the mind, the 

 whole process of development reveals to us the operation of a 

 great law of evolution and progressive change, manifested through 

 those wondrous cycles and transformations which nature seems never 

 weary of exhibiting to the earnest mind and seeking eye. 



If, finally, one might be tempted to inquire into the origin of these 

 ciliated worm-larvae themselves, we may find that speculative natural 

 history is not unprepared with a reply. We are reminded that, as the 

 early changes of egg- segmentation are not peculiar to the molluscs, 

 so neither are the veliger-stages the special belongings of that group 

 of animals. The " velum," or its representative ciliated girdles, 

 appears before us equally in the development of the echinoderms 

 or starfish group, of the worm, of the wheel-animalcule or rotifer, 

 and of the mollusc. The zoologist would further remind us that 

 these ciliary bands often remain in adult animals, and are represented 

 by certain stable possessions, such as tentacles or feelers, gills, the 

 " arms " of lampshells, and like structures. " It is probable enough," 

 says Professor Ray Lankester, " that all the ciliated bands of invertebrate 

 embryos, even of adult organisms, can be explained as derivatives of one 

 primitive organ." If this thought be fully worked out, it contains a verit- 

 able " philosopher's stone " for the zoologist ; inasmuch as it enables us 

 to account for the forms and structures of animals on a rational basis. 

 That is to say, the particular form and structure of an animal or class, are 



