10 MEMOritS OF THE NATIONAL ACADE5IY OF SCIENCES. 



mentation, i. c. the primary factors concerned in their evolution. "Weisniann in his earlier worlc 

 repeatedly as.serts that tliesc changes are dne to the direct action of external conditions togetlier 

 with natural selection. Within a few years past many naturalists have returned to a more profound 

 study of the causes of variation along some of the lines vaguely pointed out by Lamarck.' It is 

 noteworthy that Darwin changed his ^•ie\^'s somewhat in his Variation of Animals and Plants under 

 iJomesticatioa, and laid more stress on the intluence of the surroundings than in his Origin of 

 Species. 



Neither Wcismann nor other authors, however, so far as we know, have formally discussed 

 the probable mode of origin of humi)s, horns, tubercles, spines, and such outgrowths in larvie. 

 They are so marked and so manifold in their variations in form, and so manifestly related, aud iu 

 fact have so evidently been directly develoi)ed by adaptation to changes in the habits of the 

 Notodontian caterpillars and tree-feeding larva' in general tliat this group atfords fa\orable material 

 for a study of the general problem. 



Spines and prickles in animals, like tliosc^ of plants, serve to protect the organism from external 

 attack, and also to strengthen the shell or skin; tlicy are adaptive structures, and have evidently 

 arisen in response to external stiimili, either those of a general or of a cosmical nature, or those 

 resulting from the attacks of animals. It is almost an axionnitic truth that a change of habit 

 in the organism 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, " Constant extrapressure on a ])art always appears to produce 

 atroi)hy and absor])tion; occasional pressure may, and usually does, ])roduce hypertrophy and 

 tiiickeniug. All the thickenings of the cuticle are the consequences of occasional i»ressui-e, as the 

 ])ressure of shoes iu occasional walking, of tools occasionally used with the hand, and the like, 

 for it seems a necessary condition for bypertroi>hy, in most parts, that they should enjoy intervals 

 in which their nutrition may go on actively." (See Lectures on Surgical Patliology, I, p. S!>, (pioted 

 by Ilenslow, who remarks iu his suggestive Avork, "The origin of floral structures through insect 

 and other agencies," that "the 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 i)rickles of cacti and other 

 plants growing in desert or dry and sterile places are due either to defective nutrition or to " ebbing 

 vitality" (Geddes), or by others, as Mr. Wallace, to the stinuilus resulting from the occasional 

 attacks or visits of animals, especially mammals. It shmild 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 epoch, succeeding the time of river terraces. Owing to this 



' Herbert Spcnoer savs: "Tlie direct action of the medium was the primordial factor of organic evohition " (see 

 The Factor of Organic Evolution, 1>'86). Claude Bernard wrote: " The conditions of lil'o are neither iu the organism, 

 iior iu its exterual surroundings, lint iu both at once" ((luoted from .1. A. Thompson's Synthetic Summary of the 

 ]n:hicnce of the Environment upon the Organism, Proc. Eoy. I'h.vs. Soc, ix, 18.S8). Sachs renuirks: ''A far greater 

 portion of the plienouicna of life are [is] called forth by external inlluenccs than one formerly ventured to assmue" 

 (I'hys. of Plants, 1SS7, ISU, English translation). Semper claims " that of all the properties of the animal organism, 

 variability is that which may first and most easily be traced by exact investigation to its efticii'ut causes" (Animal 

 Life, etc., preface, vi). "Extern.al conditions can exert not only a very powerful selective intluence, but a transform- 

 ing one .as well, although it nnist be the more limited of the two'' (lb., S7). " No power wliich is al)le to act only as 

 a selective, and not as a transforming, inlluence can ever be exclusively put forward as the proper efficient cause — 

 (.'luaa ejiciens — of any phenomenon (lb., 404). 



•Ilenslow also ad<ls that "atro|)hy by pressure 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 be 

 the ]iersistent contact which causes a climber to thicken." 



It mayhere be noted that the results of the hypertrophy and overgrowth of the two consolidated tergites of 

 the second anteinial and mandibular segments of tlii^ Decapod Crustacea, by which the carapace has been ])roduced, 

 has resulted in a constant jiressure on the doisnl arcluis of the succeeding live cephalic and five thoracic segnients, 

 until as a result we have an atro))hy of the dorsal arches of as many as ten segments, these being covered by the 

 carapace. Audouiu early in this century enunciateil the law that iu articul.ated aninuils one part was built up at 

 the expense of adjoining portions or organs, and this is beautifully cxemplitied by the changes in the development 

 of the carapace of the embryo and larval Decapod Crustacea, and also in insects. For cxam])lc, note the change in 

 form and partial atrophy of the two hinder thoracic somites of some lieetles, as compared with the large prothorax, 

 dne probaldy to the more or less continual pressure exerted by the folded elytra aud wings. 



