396 PROGRESS OF SCIENCE IN THE CENTURY. 



wounds. Neither facts nor interpretations were 

 a-wanting a hundred years ago. 



Towards the end of the nineteenth century the 

 problem of regeneration came again to the forefront 

 of biological enquiry. The basis of fact was broad- 

 ened, and the interpretations became less vague. 



The regenerative capacity is very unequally dis- 

 tributed in the animal kingdom ; it is often exhibited 

 in regard to external parts, but rarely in regard to 

 internal parts. Its mechanism remains very ob- 

 scure, but there seems much reason to accept the 

 interpretation, which has occurred to many natu- 

 ralists from Reaumur to Weismann, but was summed 

 up in Lessona's law (1868) that regeneration tends 

 to be well-marked in those animals and in those parts 

 of animals which are in the course of natural life 

 very liable to injury. To this we may add two 

 saving-clauses, (a) always provided that the lost 

 part is of some vital importance, and (6) that the 

 wound or breakage is not fatal. The theory, the 

 Darwinian interpretation as we may call it, is, in 

 Weismann's words, that " the power of regenera- 

 tion possessed by an animal or by a part of an ani- 

 mal is regulated by adaptation to the frequency of 

 loss and to the extent of the damage caused by the 

 loss." The importance of comparing regenerative 

 processes with those of normal development is ob- 

 vious, even though both remain unread riddles. The 

 researches of Weismann and Morgan, Barfurth and 

 Bordage, Werner and Wheeler, Wolff and Miiller, 

 Loeb and Michel, are of special importance. 



In the last quarter of the nineteenth century em- 

 bryology, hitherto observational, became more defi- 

 nitely experimental. Dareste and Rauber were 

 pioneers on a line of research which has been fol- 



