Chromatin as ^'Hereditary Substance^' 335 



sperm cell of the rat. The only ground for supposing de- 

 scent in the latter case is the resemblance itself. We have, 

 consequently, no right to reason about the correspontHng 

 parts of these two organisms as though we had other })roof 

 of their kinship than that of the resemblance indicated. 



Similar needs and activities and surrounding conditions 

 tend to make organisms resemble one another. No biological 

 principle is better established than this. And surely as be- 

 tween Crithidia and a rat sperm there is much similarity of 

 need, of activity, and of environment. Both are single cells 

 of approximately equal size, and in botli a high degree of 

 locomotor ability adapted to a fluid or semi-fluid environ- 

 ment is essential ; so we are bound to recognize on purely 

 anatomical and physiological grounds and quite apart from 

 descent in any strict sense, that considerable resemblance 

 between the two might be anticipated. In other words, tlie 

 well-known and widely operative fact of parallel adaptive 

 modification in development is at least as likely to be the 

 explanation of the resemblance here as is descent. 



Appeals to recent cytological discoveries for evidence to 

 support the theory of genn-plasm continuity as the basis 

 of heredity have been altogether too unmindful of this bio- 

 logic principle of adaptive parallelism. With such a fact 

 before us, as for example that of the "practical identity" in 

 minute structure of the heart muscle of the horse-shoe crab 

 and of vertebrates ^ where there is hardly a glimmer of 

 probability that the resemblance is due to anything else 

 than adaptive parallelism, how escape recognizing that the 

 resemblance between a protozoan and a vertebrate sperm- 

 cell is probably due to the same cause? And innumerable 

 instances hardly less striking than this presented by the 

 hearts of Limulus and vertebrates could be pointed out. 



Origin of the flagellum from the chromatin of the nucleus 

 in any or many protozoans has little weight as proof that 

 the axial filament of the spermatozoan is phylogenetically 



