76 ZOOLOGY 



majority of cases nothing at all of it is known. The 

 recapitulatory interpretation of life-history was pursued with 

 such fervour during the latter half of last century that it 

 provoked a reaction, and many naturalists now speak 

 slightingly of this theory. The fact is that although re- 

 capitulation is the main factor in determining the course of a 

 development, other factors have co-operated in producing the 

 result, so that, as the late Professor Balfour said, the life- 

 history is like an old damaged record with pages missed out, 

 with other pages blurred, and with intercalation of false 

 matter. How does the historian proceed when he tries to 

 decipher such a document? He compares it carefully with 

 all the other documents of the same age which he can obtain, 

 and the true is separated from the false by the consistency of 

 the one and the inconsistency of the other. So the re- 

 capitulatory interpretation of life-history must be based on 

 Comparative Embryology, i.e. a large number of life-histories 

 must be deciphered and compared with one another before the 

 recapitulatory interpretation can be applied with any confidence. 

 But this is a slow process, and the ardent young naturalist 

 who deciphers a new life-history is usually burning with 

 impatience to apply the recapitulatory theory to it at once. 

 Hence we can easily understand that most contradictory results 

 have been obtained from the applications of this theory, 

 and hence the reaction against it. The line of research most 

 in favour at present is termed Developmental Mechanics or 

 Experimental Embryology. The object of this study is to 

 explain the developmental processes which go on in an egg, on 

 the basis of its own structure, without calling in the aid of 

 the recapitulatory theory at all. The experimenter cuts off 

 parts of the egg under observation or displaces the mutual 

 relations of the parts by pressure, &c., and then observes the 

 result. In this way he hopes to find out what are the 

 potencies of the various parts of the egg and how these parts 

 influence each other. But the progress of this research has 

 only brought out the fact that no form of egg is capable of 

 explanation by reference to its own structure alone. We find 

 that eggs differ widely from one another in their constitution. 

 In some cases, as in Worms and Molluscs, if a portion of the 

 egg be cut off the creature which develops from it will have 



