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LAW OF ACCELERATION. 47 
extreme acceleration in development, the author, in a previous paper,’ imagined 
the special means of protection often afforded to the young in such types to be 
the efficient cause. But the Marsupialia are not the most highly specialized 
mammals, nor are some of the penguins the most specialized birds, and yet both 
protect the young in pouches. Distoma and many other parasites are highly 
protected, and yet in these we find exceedingly long and complicated adaptive 
series of metamorphoses, with a high degree of protection in some cases, and in 
others more accelerated modes of development with less protection, as in Tsenia. 
Balfour in his “ Comparative Embryology ” subsequently adopted the same ex- 
planation in order to account for cases of “ abbreviated development,” some of 
which he noted, but without, however, recognizing them as due to any general 
law or tendency of development, or making quotations of Cope’s, Packard’s, or 
the author’s researches in this direction, 
The Diptera and other insects, whose larvee are placed in protected situations 
where soft foods abound, and have consequently lost their useless jaws, eyes, legs, 
and hard chitinous covering, and in some cases even the differentiated segmenta- 
tion of their ancestral forms, are very remarkable examples of acceleration in 
development. Doubtless the supply and kinds of food, and perhaps protection, 
may have had much to do with these changes, as pointed out by several ento- 
mologists. The constant correlation of habits and structure in larve is, however, 
independent of protection; and it is evident that such a limited cause could not 
have produced, as an effect, the universal tendency of acceleration. The hyper- 
metamorphosis of some insects and parasites also shows that protected habitats, 
or special maternal organs for protection, are not essential to acceleration, since 
the most complicated and indirect modes of development occur as in Sitaris, 
where the young are protected during certain stages and unprotected at others. 
Acceleration occurs whether an animal is protected or unprotected, whether 
furnished with one kind of food or another, in all sorts of habitats, and whether 
it belongs to a progressive or retrogressive series. This can be supported by 
many examples among recent animals, described by other authors than those 
specially interested in proving the truth of this law. 
With regard to the accelerated forms of shell-covered Cephalopods, they are 
usually, if belonging to the geratologous category, smaller, narrower, or less gib- 
bous, and sometimes much compressed, as in Ast. Collenoli and in Oxynoticeras, 
when compared with their immediate progenitors. The more cylindrical whorls 
of Lituites, Ophidioceras, etc., and the compressed cylindrical forms Crioceras, 
- Hamites, or Baculites, show facts of the same nature. All of these more com- 
pressed or more cylindrical shapes were obviously not adapted to the office of 
accommodating living young; they indicate in a very positive manner that 
there was less space within their whorls, and consequently less protection for 
the young, than in the more gibbous shells of congeneric normal forms. It is 
obvious that no special provision for protection existed for the young in such 
shells, or in Stephanocera refractum, or Scaphites, which did not also exist in the 
normal forms with complex modes of development. from which they were derived. 
1 Genesis of Planorbis at Steinheim, p. 29. 
