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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 embryelogists 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 
climination, 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 und 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 given 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 
