14 The American Naturalist. [January, 



Either through want of acquaintance with good examples of 

 retardation or because of a different point of view, I have not 

 been able to see any duplex action in the law of acceleration. 

 To me it is the same law of quicker inheritance which is act- 

 ing all the time in the phylum at the beginning, middle, and 

 end of its history, as will be seen by the explanation given 

 above. In Insecta 4 1 have tried to apply it to the explanation 

 of the peculiar larval forms of those animals which often pre- 

 sent retrogression through suppression of ancestral characters 

 in the young, although their adults are perfectly normal and 

 perhaps progressive. Consequently, palingenesis and coengen- 

 esis are, from my point of view, simply different forms of 

 tachygenesis, and there is no boundary or distinction between 

 them. In other words, retardation or retrogression occurs be- 

 cause of the direct action of tachygenesis upon more suitable 

 and more recently acquired characteristics which are driven 

 back upon and may directly replace certain of the ancestral 

 characters causing them to disappear from ontogenetic devel- 

 opment. 5 



ous objection to the use of cenogenesis at all, since it is from Kev6q meaning 

 strange, and was first applied by Haeckel in such a way that Uth by his ,, :it .~ 

 ments, and the derivation, it ought to be confined to types like larvae of the 

 Echinodemata Insect, etc., and parasites in which acquired characters do inter 



tachygenesis occur in a marked way might be called tachygenetic. Palingenesis 

 and palingenetic might be confined to generaliae* I forms in which the ontogeny 

 prolonged recapitulation of the phylogeny, and 



* Guides for Science Teaching, Boston Soc. Nat. Hist., No. 8. 



ii by reduction of parts is evidently included under the head of 

 tardation by Cope; thus in Origin of the Fittest, p. 353, he says that "change 

 structure during growth is accomplished either by addition or parts (actelera 

 >n) or by subtraction of parts (retardation)." So far as my experience goes in 

 e major number of ca^es, the part- <>f characters that are undergoing reduction 

 sappear according to the law of tachygenesis. They reappear in the ontogeny 

 earlier and earlier -!:,-.._, ,„. exhibit this tendency in the same way as charae- 

 -s of the progressive class, but their development i- not so complete as in ances- 

 eir development llu e is h..w , „ , t | lt , u u „t ton ... unj t! 

 >n for this. Instead of regarding this disappearance by retrogressive gradations 

 due to a tendency opposed to acceleration, is it not a tendency of the same 



