31() THE EVOLUTION THEOKY 



But there is also a physiolo<^ical proof of the meanino- of the 

 nuclear substance : and this we owe, again, to the simultaneous and 

 independent researches of two investigators, M. Nussbaum and A. 

 (Jruher, the latter working in the Zoological Institiate here (in 

 Freiburg), and at my request. They made experiments on regenera- 

 tion in unicellular organisms, and found that Infusorians which were 

 artificially divided into two, three, or four pieces were able to build 

 up a whole animal out of each piece, provided that it contained a 

 portion of the nucleus (macronucleus). The large blue trumpet- 

 animalcule, Stentor coerideus, is well suited for such experiments, 

 not only on account of its size, but because it possesses a very long 

 rosary-like nucleus, which can be easily cut two or three times. 

 When a piece is cut off which does not contain a portion of the 

 nucleus, it may indeed live for some days and swim about and 

 contract, but it is incapable of reconstructing the lost parts, and thus 

 of forming a whole animal, and it perishes. It is in the nucleus, 

 therefore, that we have to look for the substance which stamps the 

 material of the cell-body with a particular form and organization, 

 nam eh', the form and organization of its ancestors. But that is 

 exactly the conception of a hereditary substance or idioplasm (Nageli). 

 Some modern biologists deny that there is any hereditary substance 

 2^er se, and believe that the whole of the germ-cell, cell-body and 

 nucleus together effects transmission. But though it must be ad- 

 mitted that the nucleus without the cell-body cannot express inherit- 

 ance any more than the cell-body without the nucleus, this is dependent 

 on the fact that the nucleus cannot live without the cell-body ; if it 

 be removed from the cell and put, say, into water, it bursts and is 

 dissolved. But the cell-body without the nucleus lives on, though 

 of course only for a few hours or days, and its metabolism ceases 

 only when it is brought to a standstill by the failure to replace by 

 nutrition the used-up material. Thus the argument used by those 

 who deny the existence of a hereditary substance would be paralleled 

 if we denied that Man possesses a thinking substance, and maintained 

 that he thinks with his whole body, and even that the brain cannot 

 think by itself without the body. 



I am convinced that it is just as mistaken to maintain that ever}^ 

 part of an organism must contain the hereditary tendencies in the 

 same degree, or that in unicellular organisms the cell-body is as 

 important in inheritance as the nucleus (Conklin). If one feels 

 any doubt on this point, one has only to call to mind Niigeli's in- 

 ference, from the minuteness of the spermatozoon, that the hereditary 

 substance must be minimal in quantity. But even theoretically there 



