History of tlie Theory of Heredity. 19 



the changes through which the embryo passes on its road 

 from the ^gg to maturity show a wonderful parallelism 

 to the series of changes through which the organism has 

 passed during the history of its evolution from lower 

 forms. 



These results are well worth the labor they have cost, 

 and they illustrate, more clearly than any other facts in 

 biology, the common nature of all living things. They 

 do not, however, contribute directly to a clearer insight 

 into the laws of heredity. 



Ilere we are still compelled to go beyond tlie visible 

 phenomena, and to attempt by the scientific use of the 

 imagination to discover the as yet unseen relations 

 which bind tliem together. 



As we enter upon this subject it will be well to bear in 

 mind the wide difference between the end we have in 

 view — the discovery of the secondarv laws of hereditv — 

 and the attempt to understand its ultimate cause. 



Tlie pov^^cr to reproduce itself, to impress upon dead 

 inorganic matter its own distinctive properties, is one of 

 the fundamental characteristics of living matter; and 

 while we may hope that increase of knowledge may some 

 day enable us to trace the origin of this power, such an 

 attempt forms no joart of our present undertaking. 



TVe shall accept without explanation the fact that liv- 

 ing matter does thus reproduce itself, and we shall con- 

 line ourselves to the attempt to discover why the ^gg of a 

 star-lish for instance, reproduces a star-fish, and the egg 

 of a bee a bee; to discover the origin of the differences be- 

 tween the various forms of reproduction, rather than the 

 cause of what they have in common. 



The phenomena of heredity in the higher animals, as 

 well as the mechanism of ova and male cells through 

 which these phenomena are manifested, have certainly 



