x PREFACE. 
13. Acceleration in Degeneration. — Geratologous forms may, therefore, be the highest 
members of progressive series, or the terminal members of retrogressive series, and the 
stimulation of the development appears to take effect upon both progressive and retro- 
gressive characteristics ; thus producing, at the same time and in the same animal, first, the 
earlier development of some of the progressive characteristics combined with geratologous 
characteristics ; secondly, the earlier development of geratologous characteristics and their 
fusion with larval characteristics, which occasions the complete replacement of progressive 
characters, and occurs only in the extreme forms of retrogressive series, and in parasites. 
14. The law of acceleration in development seems, therefore, to express an inva- 
riable mode of action of heredity, in the earlier reproduction of hereditary characteris- 
tics of all kinds, and under all conditions. In progressive series it acts upon healthy 
characteristics, and appears to be an adaptation to favorable surroundings, and in retro- 
gressive series upon pathological characteristics, and is probably an adaptation to un- 
favorable surroundings, usually leading to the extinction of the series or type. 
15. The Three Phases of Development. — In following up series, it has been found that 
the development of ancestral forms is simple and direct (Hpacme); that of their more 
specialized descendants becomes gradually indirect (Acme), acquiring complicated inter- 
mediate or larval stages; and that of the terminal retrogressive or geratologous and 
pathological forms becomes again more or less direct (Paracme.) 
16. The introduction of adaptive larval stages into the history of individual develop- 
ment in any series appears to be due to the direct exciting action of the surroundings, 
and their absence or subsequent suppression to some physical agency, changes of habit, 
or protection, or pathological causes. All of these causes must, however, be considered 
as similar in their effect upon the young. They are stimulants, producing acceleration 
or excessively rapid development of the ancestral progressive characteristics, or of the 
retrogressive, or primitive larval characteristics inherited from the progressive forms. 
17. This agreement in the mode of development of the individual according to its 
position in the history of the group completes the correlations which exist between the 
history of the individual (ontogeny) and the history of the group to which it belongs 
(phylogeny). Using Haeckel’s nomenclature, the three periods of ontogenesis, Anaplasis, 
Metaplasis, and Cataplasis, correlate with the three periods of phylogenesis, Epacme, 
Acme, and Paracme. In addition to this general correlation, we now find that during 
the epacme of a group the development of individuals is anaplastic or progressively direct ; 
during the acme of a group, metaplastic or progressively indirect; and during the 
paracme of a group, cataplastic, or retrogressively direct. We have also found, that, in 
the history even of small groups, the epacme, acme, and paracme may often succeed one 
another in geologic time, and show similar correlations, so that we can often distinguish 
epacmic faunas, acmic faunas, and paracmic faunas in chronological succession. In 
series, also, epacmic forms, acmic forms, and paracmic forms, either in series of species 
or Varieties, may occur in geological succession in different faunas, or in zodlogical grada- 
tion in the same fauna. 
18. Law of Variation.—The action of physical changes takes effect upon an irritable, 
plastic organism, which necessarily responds to external stimulant by an internal reaction 
or effort. This action from within upon the parts of organisms modifies their heredi- 
tary forms by the production of new growths or changes, which are, therefore, adapted or 
suitable to the conditions of the habitat, and are therefore physiologically and organically 
equivalent to the physical agents and forces from which they directly or indirectly origi- 
nated. In so far, then, as causes and habits are similar, they probably produce representa- 
tion or morphological equivalence in different series of the same type in similar habitats ; 
and in so far as they are different, they probably produce the differentials which distin- 
guish series and groups from each other. 
