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THE AMEBIC AN NATURALIST [Vol. XLIV 



tion of organs. Earthworms regenerate heteromorphic 

 tails and planarians regenerate heteromorphic heads. 

 Do these peculiarities signify any reversion to a more 

 primitive condition? Double and multiple structures 

 likewise frequently appear in the regeneration of organs, 

 as, for instance, the regeneration of double heads in 

 planarians or in Lumbriculus. Do these abnormalities 

 bring any message about the normal state of things in the 

 remote past? Likewise, as DavydofY informs us, En- 

 teropneusta may regenerate double pericardial sacs or 

 double notochords. What is the morphological and his- 

 torical significance of these unusual formations'? Going 

 through a list of such irregularities and departures from 

 the normal, one may doubtless find all gradations from 

 the obvious freaks of nature down to such as may claim 

 a respectable ' ' atavistic" distinction. But the question 

 is, what will guide us in discriminating between these 

 various departures, for after all every departure in re- 

 generation originates under the special conditions of an 

 amputation. The manner in which Davydoff overcomes 

 the difficulty of this question is both characteristic and 

 interesting, and we shall return to it soon. It should be 

 pointed out, however, first that no existing theory of 

 phylogenetic significance can be invalidated to the slight- 

 est degree by the failure to regenerate on the part of 

 such structural peculiarities or characteristics as may be 

 postulated by the theory as primitive or ancestral. On 

 the other hand, any sporadic growth in the process of 

 regeneration, if by play of chance it should happen to 

 coincide with a theoretical anticipation, would become an 

 additional support to the theory. In other words, in any 

 disputed theoretical problem of animal phylogeny the 

 evidence derived from the study of regeneration must 

 by the limitations of its own nature be always of the 

 affirmative kind. It is the better part of wisdom to be- 

 lieve much too little rather than a little too much, and on 

 this account we should hesitate to rely upon evidence 

 from regeneration. 



