EMBRYOLOGY AND PAST HISTORY. 14! 



should an embryo go over such a roundabout road 

 in development, instead of proceeding to the adult 

 in the most direct manner? If possible an explana- 

 tion must be given ; and if an explanation can be 

 given in accordance with natural law, it is far prefer- 

 able to do so than to leave the whole matter in the 

 realm of the supernatural. Now the theory of evo- 

 lution has, or rather M, an explanation of this paral- 

 lel. It is not yet, perhaps, from the want of knowl- 

 edge, perfectly satisfactory in every point. But it is, 

 on the whole, so intelligible, its obscure points are 

 so rapidly being removed, and it so admirably fits 

 all the facts, that it seems hardly possible that it 

 is not the true explanation. We know that animals 

 begin their development as a single-celled ovum ; 

 and, according to the descent theory, a single-celled 

 animal was a stage very near the starting-point of 

 life in the past ages. We know that animals do in- 

 herit the characteristics of their ancestors ; not only 

 those of their parents, but of more remote genera- 

 tions. There is abundance of evidence for the fact 

 that these inherited characteristics may be developed 

 in the offspring at an earlier period than they appear 

 in the parent, and thus peculiarities of the adult 

 may slowly become those of younger stages. Now, 

 with all of these facts in mind, it is very easy to see 

 why it is that in developing from the single-celled 

 ovum to the adult, the embryo should, by the ac- 

 cepted law of heredity, pass over the same road 

 which was travelled by its ancestors in developing 

 from the unicellular animal to its present condition. 

 Heredity, then, that mysterious law which we know 



