Packard.] 
504 
[Feb. 19, 
Darwin changed his views somewhat in his Variation of Animals 
and Plants under Domestication, and laid more stress on the influ- 
ence of the surroundings than at first. 
Neither Weismann nor other authors, however, so far as we know, 
have formally discussed the probable mode of origin of humps, 
horns, tubercles, spines and such outgrowths in larvae. They are 
so marked and so manifold in their variations in form, and so man- 
ifestly related and in fact have so evidently been directly developed 
by adaptation to changes in the habits of the Notodontian caterpil- 
lars and tree-feeding larvae in general that this group affords favor- 
able material for a study of the general problem. 
Spines and prickles in animals, like those of plants, serve to 
protect the organism from external attack, and also to strengthen 
the shell or skin ; they are adaptive structures, and have evidently 
arisen in response to external stimuli, either those of a general or a 
cosmical nature, or those resulting from the attacks of animals. 
It is almost an axiomatic truth that a change of habit in the organ- 
ism precedes or induces a change of structure. 
What has caused the enlargement and specialization of certain 
of the piliferous warts? As remarked by Sir James Paget : “Con- 
stant extra-pressure on a part always appears to produce atrophy 
and absorption ; occasional pressure may, and usually does, produce 
hypertrophy and thickening. All the thickenings of the cuticle are 
the consequences of occasional pressure ; as the pressure of shoes 
in occasional walking, of tools occasionally used with the hand, and 
the like ; for it seems a necessary condition for hypertrophy, in most 
parts, that they should enjoy intervals in which their nutrition may 
go on actively.” (See Lectures on Surgical Pathology, i, p. 89, 
quoted by Henslow, who remarks in his suggestive work : “ The 
origin of floral structures through insect and other agencies” that 
“the reader will perceive the significance of this passage when recall- 
ing the fact that insects’ visits are intermittent.” 1 ) 
1 Henslow also adds that “ atrophy by pressure and absorption is seen in the growth 
of embryos; while the constant pressure of a ligature arrests all growth at the con- 
stricted place. On the other hand, it would seem to be the persistent contact which 
causes a climber to thicken.” 
It may here be noted that the results of the hypertrophy and overgrowth of the two 
consolidated tergites of the second antennal and mandibular segments of the Decapod 
Crustacea, by which the carapace has been produced, has resulted in a constant pres- 
sure on the dorsal arches of the succeeding five cephalic and five thoracic segments, 
until as a result we have an atrophy of the dorsal arches of as many as ten segments, 
these being covered by the carapace. Audouin, early in this century, enunciated the 
law that in articulated animals one part was built up at the expense of adjoining por- 
tions or organs, and this is beautifully exemplified by the changes in the development 
