LAWS OF HEEEDITY. 137 



few others ; yet only one exhaustive treatise exists in 

 English on this subject. 



Although open to the charge of telling a thrice-told 

 tale, the author puts forth the following remarks, 

 which embody some of the more recent labours of 

 continental workers ; but they are intended chiefly for 

 the use of those who may not have leisure or inclina- 

 tion to consult such in their original forms. For those 

 who look for pure histology and complete embryology, 

 references may be found in the appended bibliographic 

 list of authors. 



The science of embryology as it now exists is com- 

 paratively of a recent date. Much of its present value 

 and importance consists in its bearing on phylogeny, 

 or the history of races. It seems now to be a recog- 

 nised truth that the embryos of the higher animals, 

 though not resembling the adult forms of the lower 

 animals, nevertheless show a close similitude to the 

 embryos of the latter. Physiologists are multiplying 

 proofs in this direction almost daily. 



Prof. Balfour points out that these phenomena have 

 an explanation in the somewhat antagonistic principles 

 of heredity and variation. The first law enunciates 

 that " the characters of an organism at all stages of 

 its existence are reproduced in its descendants at 

 corresponding stages." The second law asserts " that 

 offspring never exactly resemble their parents." By 

 the common action of these two principles, continuous 

 variation from a parent type becomes a possibility, 

 since every acquired variation has a tendency to be 

 inherited. 



Again, " each organism reproduces the variations 

 inherited from all its ancestors at successive stages in 

 its individual ontogeny, which correspond with those 

 at which the variations appeared in its ancestors."* 



It will be interesting, therefore, to ask if any and 



* Prof. F. M. Balfour, 'Treatise on Comparative Embryology,' 

 Introd., vol. i, p. 3. 



