1890 .] 
515 
[Packard. 
rolled up at the end, it is certainly very suggestive that these are 
situated on top of the loop made by the caterpillar’s body during 
progression, the first pair arising from the second, and the hinder 
pair from the fourth abdominal segment. 
It seems, therefore, that the humps or horns arise from the most 
prominent portions of the body, at the point where the body is most 
exposed to external stimuli ; and the force of this is especially seen 
in the conspicuous position of those tubercles which are volunta- 
rily made to nod or so move as to frighten away other creatures. 
Perhaps the tendency of these segments to loop or hump-up has 
had a relation of cause and effect in inducing the hypertrophy of 
the dermal tissues entering into the composition of the tubercles 
or horns. 
Analogous positions are in the vertebrates utilized, as in spiny, 
osseous fishes, or the sharks, the horned Amphibia, or horned rep- 
tiles and horned mammals. The prominence of the foundation 
parts, from which the tubercles arise, may lead to a determination 
of the blood toward such places and thus in well-fed or over-fed 
(possibly under-fed individuals) induce a tendency to hypertrophy 
which once set up in early generations led to the production of in- 
cipient humps which became more developed as they proved useful 
and became preserved in this or that form by natural selection. On 
the other hand, the hypertrophy of certain piliferous warts would 
tend to cause an arrest of development or a tendency to atrophy 
in the piliferous warts of adjoining segments. And in like man- 
ner may the simple setae have become hypertrophied on account of 
their great utility as deterrent organs, and become wonderfully 
modified in this and that direction in such and such forms, until 
they became in recent geological times the common and normal in- 
heritance, not only of scattered species but of certain genera in 
scattered families, and even of entire families. 
It is to be observed, as one will see by referring to the special 
larval histories and the recapitulations which we have appended, 
that, as in the species of Schizura,the evolution or hypertrophy of 
the movable or nutant tubercles begins in the third stage, at about 
the time when the young caterpillars leave their common birth- 
place on the under side of the leaf and seek more conspicuous feed- 
ing grounds on the outer edge or on the upper side of the leaf, 
where they are exposed to the visits of ichneumons, or Tachinae, 
or carnivorous Hemiptera, or to the onset of open-mouthed insec- 
