C. E. Beecher — Origin and Significance of Spines. 333 



developed out of structures which are normally of vital physio- 

 logical importance. 



An animal or plant having spines and living in a favorable 

 environment, involving freedom of motion for animals, and 

 abundance of nutrition without extremes of temperature or 

 dryness for both animals and plants, will, it is believed from 

 the discussions and analyses of spine genesis in its various 

 phases, develop these features in most instances, without the 

 sacrifice of organs and structures having important physiolog- 

 ical and motor functions. Thus, ordinarily, among animals it 

 is found that spines arise as excrescences or outgrowths of 

 exoskeletal or epidermal tissues, without seriously affecting the 

 function of the organ or organs upon which they are located. 

 Such cases may clearly belong to the most progressive series, 

 and in fact usually occur there. 



On the other hand, if it is found that a leg, a wing, a digit, 

 or other organ is developed into a spine, this is always accom- 

 plished by a process of retrogression, resulting in the greater 

 or lesser suppression of the part in question. It is also seen 

 that this kind of spine occurs most frequently in retrogressive 

 series or in others showing arrested development, and the 

 necessary interpretation seems to be either that the environ- 

 ment is or has been unfavorable, at least so far as the particu- 

 lar organ or set of organs is concerned, or that the vital power 

 has declined. Both influences are intimately associated, and 

 the latter is often the direct result of the former. 



The stunting effects of aridity and barren soil on our com- 

 mon plants is familiar to all. Among the plants of the desert 

 is found every evidence of similar stunting combined with 

 adaptations to resist the unfavorable conditions of deficient 

 water supply, excess of radiation, etc. The diminution in size 

 applies not only to stature, but to the leaves and branches, 

 especially the parenchymatous tissues or parts of the plant 

 engaged in aereal assimilation. Consonant with these changes, 

 the drought and other conditions produce a hardening of the 

 mechanical tissues, which is of great aid in resisting the extreme 

 heat and dryness of the desert. Sometimes a deposit of wax 

 affords a similar protection. 



The reduction of the leaves takes place in various ways. 

 They may simply become smaller in every dimension and 

 finally be reduced to mere scales, or an aphyllous condition 

 may be established. They may grow narrower and narrower 

 until only the hardened veins or midrib remains ; or leaves may 

 be developed only for a short time, and, in the case of com- 

 pound leaves, after the shedding of the leaflets, a spiniform 

 leaf axis remains, as in Astragalus Tragacantha** (figures 55, 

 56). The suppression of branches tends towards the same end ; 



Am. Jour. Sci. — Fourth Series, Yol. YI, No. 34.— October, 1898. 

 23 



