54 CHAPTERS O.V EVOLUTION. 



the lancelet presents distinct affinities with the sea-squirts (Fig. 14),. 

 or Tunicates, which belong to the molluscoid type (see table, page 47),. 

 and the commoner species of which may be compared each to a 

 veritable bag, or " leather bottel," firmly attached to rocks, shells, 

 and like objects. These likenesses, to be more fully discussed 

 hereafter (see Chapter IX.), are seen, not merely when the structure 

 of the adult sea-squirt and lancelet are compared, 

 but are still more clearly discernible when the 

 development of the two animals is studied. The 

 lancelet, in short, resembles a fish, or lower verte- 

 brate, whose development has been arrested, so to 

 speak ; and it is equally interesting to discover that 

 there exist certain sea-squirts which, in their special 

 features, approach very nearly to the lancelet, and 

 in which the " notochord " long supposed to be 

 the special possession of the young of vertebrate 

 animals remains, as in the lancelet, persistent 

 throughout life. Thus the lancelet remains before 

 us, constituting, in every sense of the term, a link 

 between vertebrates and invertebrates. It agrees 

 wholly neither with the highest type nor with the 

 molluscoids or sea-squirts themselves, but exhibits 

 a series of characters strictly intermediate between the two types. 

 We may readily enough understand, on these grounds, why this little 

 clear-bodied animal, which at first was regarded as a worm, and then 

 as a kind of slug, should, from the peculiarity of its position as the 

 apparent root of the vertebrate type, have been styled, " next to man,, 

 the most important vertebrate." 



As Professor Huxley has pertinently remarked, "in 1859 there 

 appeared to be a very sharp and clear hiatus between vertebrated and 

 invertebrated animals, not only in their structure, but, what was more 

 important, in their development. I do not think that we even 

 yet know the precise links of connection between the two ; but the 

 investigations of Kowalewsky and others upon the development of 

 Amphioxus and the Tunicata prove, beyond a doubt, that the dif- 

 ferences which were supposed to constitute a barrier between the 

 two are non-existent. There is no longer any difficulty in un- 

 derstanding how the vertebrate type may have arisen from the 

 invertebrate, though the full proof of the manner in which the 

 transition was actually effected may still be lacking." For these 

 weighty reasons, the vertebrate type, in the tabular view of the types 

 of animal life (page 47), is represented as having its root laid 

 within the sea-squirt or " tunicate " line of descent. 



If at random we selected other types of animal life, we should 

 similarly discover that they exhibit more or less distinct relationships. 



