142 BIOLOGICAL LECTURES. 



It has been called the " law of Tachygenesis, or accelerated 

 development," and is known as an isolated fact to most embry- 

 ologists under the name of " abbreviated development." It is 

 constantly noted by investigators, but, in consequence of their 

 failure to follow out their observations in serial succession 

 according to the natural order of phylogeny, they have not 

 realized that extreme cases of abbreviation are only exag- 

 gerated phases of a law of heredity acting more or less in all 

 forms. 



In tracing phylogenesis by the correspondences of the 

 younger stages and by the gradations of the adult stages, it 

 has been found that successive, genetically connected forms 

 tend to inherit the characters of their ancestors at earlier 

 stages than those in which they first appeared in those ances- 

 tors. The law as observed by breeders and formulated by 

 Darwin for existing animals and plants is that characters of 

 parental forms tend to be inherited at the same time or earlier 

 in descendants. In phylogeny the result of this is cumulative, 

 and in favor of continuously increasing acceleration. Thus, the 

 latest acquired characteristics of any ancestral form appear ear- 

 lier, and ever earlier, in its descendants. In this way room is 

 made for still other acquired characters that are introduced later 

 in the lives of these same forms. In its normal course in phy- 

 logeny a characteristic takes up in succession different places 

 in the early ephebic, then passes into the neanic substages, 

 and finally comes into actual contact with the more invariable 

 nepionic characters. Then, according to the law of Tachygen- 

 esis, it is, as a rule, crowded out of the ontogeny and ceases to 

 be recapitulated. Sometimes, as shown in a yet unpublished 

 paper by Grabau, among Gasteropoda an apparently trivial char- 

 acter may pass this ordeal and actually invade the later substage 

 of the embryo. As a rule, however, the characters that arise 

 in the smaller phyla perish out of the ontogeny when they meet 

 the more persistent nepionic and embryonic characters. 



Physiology, Dec. 29, 1897, published in Ann. of Botany, vol. xii, December, 1898; 

 and Jackson, "Localized Stages," op. cit., p. 138 (read Nov. 2, 1898, published 

 April, 1899), notices this law in plants. These all, except Jackson's, represent 

 independent rediscoveries of the law of Tachygenesis. 



