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facts. Hyatt (31) says: "All modifications and variations in progressive 

 series tend to appear first in the adolescent or adult stages of growth, 

 and then to be inherited in successive descendants at earlier stages ac- 

 cording to the law of acceleration, until they either become embryonic or 

 are crowded out of the organization and replaced in the development by 

 characters of later origin." A more concise statement of the law is as 

 follows : "The substages of development in ontogeny are the bearers of 

 distal characters in inverse proportion and of proximal characters in 

 direct proportion to their removal in time and position from the proto- 

 conch, or last embryonic stage" (31). 



According to the definitions just quoted, acceleration involves not 

 only the omission of characters, in some cases (and this is the only sort 

 of acceleration that most embryologists seem to recognize), but it involves 

 also .condensation without omission, by crowding more into a given por- 

 tion of the ontogeny, or again by "telescoping" of characters, as Grabau 

 (25) calls it. so that characters that originally appeared in succession, 

 come to appear simultaneously. In other words acceleration may be by 

 elimination, by condensation, without change in the order of appearance of 

 characters, or. third, by telescoping. The latter is condensation with 

 change in the order of appearance, or as commonly expressed, unequal 

 acceleration. It is probable that paleobiologists have erred in giving too 

 much emphasis to the principle of earlier inheritance, involved in the 

 law. just as embryologists and morphologists have erred in entirely neg- 

 lecting this phase of inheritance. As conceived by the paleobiologist, the 

 law of acceleration is an explanation of recapitulation, as well as an ex- 

 planation of the failure to recapitulate. 



Another factor in inheritance has been gh r en the name of retardation 

 by Cope (15). By the operation of this factor, characters that appear 

 late in the ontogeny may disappear in descendants because development 

 terminates before the given character is reached. In this way. it is con- 

 ceived, the ontogeny may be shortened and simplified, and many ances- 

 tral characters lost entirely. The result of the continued operation of 

 retardation would be retrogression. That is. the given form, if it con- 

 tinued to repeat the remoter ancestral stages in the early part of its 

 ontogeny, and continued at the same time to drop off the later ancestral 

 stages, by failing to proceed far enough in its development, would ulti- 

 mately come to resemble the remote rather than the nearer ancestors. 

 Manifestly the retarded forms do not recapitulate the lost characters, so 



