394 NOTES AND COMMENTS [December 



and differences are due to real transmission of heritable qualities, and 

 how far to similarity in the induced " modifications " ; there is the 

 analysis of the inheritance by statistical and experimental methods, the 

 biggest result of which has been Galton's law ; and there is the at 

 present almost unassailable problem of conceiving how the heritable 

 qualities work their way into realisation during the process of develop- 

 ment — a problem that leads us away from the strict problem of 

 heredity to that of " the principles of development." In short, we 

 hardly think that the serious student of heredity has ever thought that 

 he was facing one problem to which it might be expected some day to 

 find one answer. In any case, we cannot admire the ingenious 

 observer's phraseology when he says that " what we call heredity is 

 composed of very heterogeneous constituents." He speaks of the so- 

 called theories of Eimer as " nothing but a play on words," but might 

 not Loeb strengthen his case by taking his own words more seriously ? 



The particular problem which Professor Loeb discusses is that of the 

 tiger-like markings in the yolk sac of the embryo of the fish called 

 Fundulus — a subject in regard to which he has previously published 

 results. The origin of the coloration is as follows : — black and red 

 chromatophores are found on the surface of the yolk sac ; they gradually 

 creep upon the blood-vessels and ensheath these, exhibiting chemo- 

 tropism clue especially to the oxygen of the blood, or stereotropism 

 (another brave word), or both. " The heredity of the markings is, 

 therefore, in this case determined by a stimulus which the blood- 

 vessels exert upon another tissue, namely, the chromatophores. Both 

 tissues are formed rather independently of each other, but from the fact 

 that the chromatophores must creep upon the blood-vessels, and that 

 the latter have a hereditary arrangement, the marking becomes heredi- 

 tary too. This contradicts those theories of heredity which try to 

 derive all the peculiarities of the animal from corresponding peculiarities 

 of the sexual cell, for instance, Weismann's theory." But this is 

 lamentable confusion ; for no one surely has supposed that there are 

 not analysable immediate conditions operative at every stage of 

 development : the point of Weismann's theory is that the inherited 

 organisation determines the particular occurrence and sequence of these 

 conditions, and is thus the primary though not the immediate cause of 

 the results. 



Loeb gives an interesting figure of the tail of an embryo, in which 

 the chromatophores are seen to have crept upon the median artery 

 while the vein remains free. This suggests that the oxygen of the 

 blood may be one of the causes that force the chromatophores to creep 

 upon the blood-vessels. But this is not the whole reason, for wherever 

 a vein is isolated they creep upon it too. Moreover, the back of the 

 embryo is coloured black by pigment cells which follow the brain and 

 the spinal cord. 



The observations are interesting, but they appear to us to have to 



