190 CHAPTERS ON EVOLUTION. 



The method of disappearance of the gills and their arches is as 

 reasonably detailed, when Darwin states that, " in order to under- 

 stand the existence of rudimentary organs, we have only to suppose 

 that a former progenitor possessed the parts in question in a perfect 

 state, and that under changed habits of life they became greatly 

 reduced, either from simple disuse, or through the natural selection 

 of those individuals which were least encumbered with a superfluous 

 part, aided by the other means previously indicated." 



Another fact of interest, derived from our studies in development, 

 relates to the position of the sea-squirt larva and of the lancelet as 

 together constituting " links " which -bridge the gulf between the 

 Invertebrates and their higher backboned neighbours the vertebrate 

 animals. Only when we think of the apparently great gulf fixed 

 between the fishes as the lowest Vertebrates and all invertebrate 

 forms, can we realise the gain to evolution of the knowledge which 

 shows how the development of the sea-squirt and that of the lowest 

 Vertebrate run in parallel lines. Such a correspondence in de- 

 velopment, and the discovery of the possession by sea-squirts of 

 the " notochord " long thought to be the exclusive possession of the 

 Vertebrates constitute together a veritable tower -of strength for the 

 evolutionist, whence he may survey a formidable gap supplied, and a 

 " missing link" satisfactorily brought to light. 



It should lastly be noted that the facts brought to light by a study 

 of the early stages in the development of animals, may be regarded 

 as being thoroughly in favour of the theory of Evolution. Professor 

 Allen Thomson, in his presidential address to the British Association 

 (1877), stated the latter fact forcibly when he said, "I consider it 

 impossible, therefore, for any one to be a faithful student of embry- 

 ology, in the present state of science, without at the same time 

 becoming an evolutionist." These are weighty words, but they are 

 fully justified by the circumstances of the case to which they apply. 

 And no less apt are the terms in which the same authority in matters 

 embryological further alludes to the support received by evolution 

 from daily life histories of living beings. " If," says Professor 

 Thomson, "we admit the progressive nature of the changes of 

 development, their similarity in different groups, and their common 

 characters in all animals nay, even, in some respects, in both plants 

 and animals we can scarcely refuse to recognise the possibility of 

 continuous derivation in the history of their origin." 



