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LAW OF ACCELERATION. 45 
superior adaptability, interfere with the development of the less useful ancestral 
stages, and thus tend to replace them. The necessary corollary of this process 
would be the acceleration of the previously existing nealogic stages in direct pro- 
portion to the number of new characters successively introduced into the direct 
line of modification during the evolution of a group. 
The importance of this law becomes more apparent when consideration is 
claimed for it as a working hypothesis for the explanation of such obscure prob- 
lems as occur among insects. The complicated metamorphoses of the Hymen- 
optera, Diptera, and some Coleoptera, for example, in which feetless and headless 
larvee appear, can be attributed to acceleration, like the more normal examples 
among fossil Cephalopoda. They illustrate the suppression of ancestral thysanu- 
riform stages, which when present in the active larve of lower orders indicate 
that all insects were derived from some ancestor possibly similar to the adult 
of such forms as Lepisma or Campodea. This gives new interest to the theoreti- 
cal views of Brauer and Sir John Lubbock, who first pointed out the nepionic 
characteristics of the adults among Thysanura. 
It seems to us equally applicable to the explanation of the medusaless meta- 
morphoses of the fresh-water Hydra, as compared with the marine Tubularia in 
which the medusa stage is prevalent, and also to the accelerated development 
of the pelagic meduse, Geryonia and others, in which the hydra-like stage has 
vanished. 
In Tenia, also, the earlier stages are so accelerated that the secondary sac, 
furnished with cutting blades, worked by special muscles, and used for digging 
through the tissues, is still called an ovum by many naturalists, though it is mor- 
phologically the remnant of an active form producing the young Txnia by an 
involution or bud from its walls. The ancestors of Tania must first have 
acquired a cercaria or nurse form with cutting blades, and then, the evolution 
having reached its highest progressive acme, the reverse process of resorption 
through acceleration began. The constant exercise of the blades by the cer- 
caria. and the use of a horny case for the ovum caused these to be retained, while 
the other characteristics of the cercarian stage disappeared, or else like the blades 
became more or less fused with the ovarian stages. 
Perhaps the most. remarkable instance of the loss of progressive characters 
correlating with a highly accelerated mode of development is man himself;! 
and his example will serve a good purpose in making clear what we mean by 
a geratologous retrogression, which is often evidently due to a great change in 
habits, bringing about specialization in certain parts, enlarging and prematurely 
developing them at the expense of many of the normal progressive characters of 
the ancestral type. The Caucasian type,in losing the prognathism of the An- 
thropoids, which is certainly a highly specialized characteristic of the adult forms 
among the apes, has in a morphological sense made a step backwards instead of 
forwards. The larger size of the brain as compared with the lower part of the 
face and jaws is also an embryonic characteristic of all the Vertebrata, even 
1 See Cope, Origin of the Fittest, pp. 11, 12, 147, 148, chaps. viii., ix., also Haeckel, Gen. Morphologie, 
II. p. 446, and Anthropogenie, for similar views. 
