16 
MEMOTKS OF THE NATIOIn'^AL ACADEMY OF SCIENCES. 
mentation, i. e., the priniiiry factors concerned in tlieir evolution. Weisnuinn in his earlier work 
repeatedly asserts that these changes are due to the direct action of external conditions together 
with natural selection. AVithin a few years ])ast many naturalists have returned to a more profound 
study of the causes of variation along some of the lines vaguely pointed out hy Lamarck.* It is 
noteworthy that Darwin changed his view's somewhat in his A'ariation of Animals and Plants under 
Domestication, and laid more stress on the intiueuce of the surroundings than in his Origin of 
Species. 
Neither Weisinann nor other authors, however, so far as we know, haA^e formally discussed 
the i^robable mode of origin of humps, horns, tubercles, sx)ines, and such outgrowths in larva3. 
They are so marked and so manifold in their variations in form, and so manifestly related, and in 
fact have so evidently been directly developed by adaptation to changes in the habits of the 
Notodontian caterpillars and tree-feeding larvai in general that this group affords favorable material 
for a study of the general iwoblem. 
Spines and ])rickles in animals, like those of idants, serve to protect the organism from external 
attack, and also to strengthen the shell or skin; they are adaptive structures, and have evidently 
arismi in response to external stimuli, either those of a general or of a cosmical nature, or those 
resulting from the attacks of animals. It is almost an axiomatic truth that a change of habit 
ill the organism precedes or induces a change of striictiiic. 
What has caused the enlargement and specialization of certain of the piliferous warts ? As 
remarked by Sir James Paget, Constant extrapressiire on a part always appears to produce 
atrophy and absorption; occasional iiressnre may, and usually does, produce hypertroiJiy and 
thickening. All the thickenings of the cuticle are the consequences of occasional pressure, as the 
pressure of shoes in occasional AA'alking, 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 I^ectures on Surgical Pathology, I, p. 89, (pioted 
by llenslow, who remarks in his suggestive work, ‘‘The origin of lioral structures through insect 
and other agencies,’’ that ‘Hhe reader will perceive the significance of this passage when recalling 
the fact that insects’ visits are intermittent.”^) 
It is now assumed by some naturalists that the thorns, spines, and prickles of cacti and other 
jAlaiits growing in desert or dry and sterile places are due either to defective nutrition or to “ebbing 
Autality” (Geddes), or by others, as Mr. Wallace, to the stimulus resulting from the occasional 
attacks or visits of animals, especiailj^ mammals. It should be borne in mind that the great deserts 
of the globe are of quite recent formation, being the result of the desiccation of interior areas of 
the continents, late in the Quaternary ejioch, succeeding the time of river terraces. Owing to this 
■* Herbert Spencer says: The direct action of the medium was the primordial factor of organic evohitiou ” (see 
I’lio Factor of Organic K volution, 1880). Claude Bernard wrote: The conditions of life aro neither in the org.anism, 
nor in its external surronudinga, hut in both at once” (quoted from J. A. Thompson’s Synthetic Summary of the 
Jnllneuee of the Environment upon the Organism, Proc. Eoy. Phys. Soe., ix, 1888). Sachs remarks: far greater 
portion of the phenomena of life are [is] called forth hy external infinences than one formerly ventured to assume” 
(Pliys. of Plants, 1887, llU, English translation). Semper claims that of all the properties of the animal organism, 
variability is that which may first and most easily he traced hy exact investigation to its elhcicnt causes” (Animal 
Life, etc., preface, vi). ‘^External conditions can exert not onlya very powerful selective inlhicnce, but a transform- 
ing one as well, although it must he the more limited of the two” (Ib., 37). ^^Xo jiower which is able to act only as 
a selective, and not as a transforming, influence can ever he exclusively put forward as the proper efficient cause— 
ejjiciens — of any phenomenon (Ib,,401). 
^Heuslowalso adds that “atrojihy hy jircssure and absorption is seen in the growth of embryos, while the 
constant pressure of a ligature arrests all growth at the constricted place. On the other hand, it would seem to he 
the persistent contact which causes a climber to thicken.” 
It mayhere he 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 iu a constant pressure on the dorsal arches of the succeeding five cephalic and live thoracic segments, 
until as a result wo have an atrophy of the dorsal arches of as many as ton segments, these being covered hy the 
carapace. Andouiu early in this century enunciated the law that iu articulated animals one part was built up at 
the expense of adjoining portions or organs, and this is beautifully exemplified hy the changes in the development 
of the carapace of the embryo and larval Decapod Crustacea, and also in insects. For example, note the cliange in 
form and partial atrophy of the two hinder thoracic somites of some beetles, as compared with the largo prothorax, 
due probably to the more or less continual pressure exerted hy the folded elytra and wings. 
