232 



CHAPTERS ON EVOLUTION. 



the body becomes segmented, and the tentacle which, under the 

 name of the operculum (Fig. 158, o\ is destined to form a stopper 

 to the mouth of the tube, may likewise be discerned. At this stage, 

 with its segmented body, the young tube-dwelling worm resembles 

 the permanent condition of its free-living neighbour of the sand 

 (Fig. 156). Hence, when we discover that the tube-dweller finally 

 secretes a tube, and lodging its body therein, becomes a stationary 

 form, we conclude, rationally enough, that both kinds of worms have 

 arisen from one common stock, and that the tube-dwellers represent 

 the more modified race of the two groups ; whilst they likewise may 



be regarded as "degraded" forms 

 when compared with their free- 

 living neighbours (Fig. 156). 



We have thus had presented 

 to view a series of developments 

 extending from those of the mol- 

 luscs, through the " lampshells," 

 and finally ending with that of the 

 worms themselves. Is there evi- 

 dence at hand to show that some- 

 thing more than a theoretical con- 

 ception of the connection between 

 these apparently dissimilar forms 

 is a warrantable thought? The 

 answer to such a question depends 

 ; on the credence we place on what 

 development teaches. If the truth 

 of the axiom that " development 

 repeats descent " be not admitted, 

 it is worse than useless to invite 

 comparison between the larva of a chiton and that of a worm. Unless 

 the mind has been prepared to discover in development the shifting 

 and progressive past history of a species, there can be no benefit of 

 an intellectual kind in comparing the likeness of the young brachiopod 

 with the early stages of the worm. But, conversely, when it is admitted 

 that all development is meaningless unless some idea of its use, 

 purport, or cause is afforded, and when in the study of the phases 

 of an animal's growth we are led to see prospects of tracing its past 

 evolution, the likenesses and analogies of development become forcibly 

 plain and valuable. Primarily, it may be said that a very large part 

 of the reasonableness of evolution depends on its rational interpre- 

 tations of development. Without development and its lessons, 

 evolution would be well-nigh unprovable. Conversely, without the 

 idea of evolution, the development of animal or plant is a meaningless 

 piece of natural transmogrification and change. 



FIG. 158. SERPCLA. 



