LAW OF ACCELEKATION. 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- 

 op tera, Diptera, and some Coleoptera, for example, in which feetless and headless 

 larvae 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 larvae 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 naepionic 

 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 medusa?, Geryonia and others, in which the hydra-like stage has 

 vanished. 



In Taenia, 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 Taenia by an 

 involution or bud from its walls. The ancestors of Taenia 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; 1 

 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 tj'pe, 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. 



