GENESIS, HEREDITY, AND VARIATION. 361 



The hypothesis of physiological units, as set forth in preced- 

 ing chapters, appears less incompetent: reproduction of the 

 lost part would seem to be a normal result of the proclivity 

 towards the form of the entire organism. But now what are 

 we to say when, instead of being cut off transversely, the 

 tail is divided longitudinally and each half becomes a com- 

 plete tail? What are we to say when, if these two tails are 

 similarly dealt with, the halves again complete themselves; 

 and so until as many as sixteen tails have been formed? 

 Here the hypothesis of physiological units appears to fail 

 utterly ; for the tendency it implies is to complete the specific 

 form, by reproducing a single tail only. 



Various annulose animals display anomalies of develop- 

 ment difficult to explain on any hypothesis. We have 

 creatures like the fresh-water Nats which, though it has ad- 

 vanced structures, including a vascular system, branchiae, 

 and a nervous cord ending with cephalic ganglia, nevertheless 

 shows us an ability like that of the Hydra to reproduce the 

 whole from a small part: nearly forty pieces into which a 

 Nais was cut having severally grown into complete animals. 

 Again we have, in the order Polychcetce, types like Myrianida, 

 in which by longitudinal budding a string of individuals, 

 sometimes numbering even thirty, severally develop certain 

 segments into heads, while increasing their segments in 

 number. In yet other types there occurs not longitudinal 

 gemmation only, but lateral gemmation : a segment will send 

 out sideways a bud which presently becomes a complete 

 worm. Once more, Syllis ramosa is a species in which the 

 individual worms growing from lateral buds, while remaining 

 attached to the parent, themselves give origin to buds; and 

 so produce a branched aggregate of worms. How shall we 

 explain the reparative and reproductive powers thus exempli- 

 fied? It seems undeniable that each portion has an ability 

 to produce, according to circumstances, the whole creature 

 or a missing part of the creature. When we read of Sir J. 

 Dalyell that he " cut a Dasychone into three pieces ; the 



