MEMOIRS OF THE NATIO^STAL ACADEMY OF SCIENCES. 17 



widespread change in tlie eiivii-oiiiiieiit, involving a drying iii> of the soil, much of it alkaline, the 

 direct influence on plant life must have been profound, as regards their protective defenses, aud 

 after S])ines began to develop one can well understand how their shapes should have been regulated 

 for each species and preserved by the set of minor factors which pass current under the term 

 "natural selection." 



Animals may also, in some cases, have developed spines in response to a change of environ- 

 ment. If we glance over the epochs of paleontological history we shall see that at certain periods 

 trilobites, brachiopods, ammonites, and perhaps otlier groups showed a tendency to become tuber- 

 culated, spiny, or otherwise excessively ornamented. These periods must have been characterized 

 by great geological changes, both of the relative distribution of land and water and iierhaps of 

 climate and soil. Among the brachiopods, more spiny species occur in the Carboniferous period 

 than iu the earlier Paleozoic times,' Among the trilobites, although in Paradoxides and in other 

 genera the gense and sides of the segments are often greatly elongated, we only find forms with 

 long dorsal spines at the close of the Silurian and during the Devonian.^ There are no such S])iny 

 forms of ammonites as in the uncoiled Cretaceous Crioceras,' etc. 



These types, as is well known, had their period of rise, culmination, and decline, or extinc- 

 tion, and the more spiny, highly ornamented, abnormal, bizarre forms appeared at or about the 

 time when the vitality of the type was apparently declining. Geddes claims that the splines of 

 plants are a proof of ebbing vitality. Whether or not this was the case with the types of animal 

 life referred to, whether the excess of ornamentation was due to excess or deficiency of food, it is 

 not improbable that the appearance of such highly or grotesquely ornamented forms as ceitain 

 later brachiopods, trilobites, and ammonites was the result of a change in their environment during 

 a period when there were more widespread and profound changes in physical geography than had 

 perhaps previously occurred. 



If the tendency to the production of spines in past geological times was directly or indirectly 

 due to a change in the milieu, and if plants when subjected to new conditions, such as a transfer 

 to deserts, show a tendency to the growth of thorns, or if those which are constantly submerged 

 tend to throw out ascending aerial root^,'* or if, like epiphytes, when growing iu mid-air, they throw 

 out descending aerial roots, I have thought it not improbable tliat tubercles, humps, or spines may 

 have in the first place been developed in a few generations, as the result of some change iu the 

 environment during the critical time attending or following the close of the Paleozoic or the early 

 part of the Mesozoic age, the time when deciduous trees aud flowers probably began to appear. 



I have alwaj'S regarded the Bombyces, or the superfamily of silkworm moths, as a very 

 ancient one, which has lost many forms by geological extinction. We thus account for the many 

 gaps between the genera. Both the larv;e and the moths differ structurally far more than the 

 genera of Geometrids aud of Noctuidiie, and the number of species is less. The two latter families 

 probably arose from the great speci;dizatioa of type in Tertiary times; while evidently the great 



' Althougli tbere are spiny brachiopods in the Silurian, thoy become more common in the Devouian (e. g., Atri/pa 

 hi/slrix, Chonetes scitula, C. coroiiala, C. muricala, I'rodnctella hirsiila, P. hjisti-iciila, P. rarhpiva, an 1 Slrophacosia 

 prodiictoidcs), and are apparently more numerous in the Carboniferous formation (e. g., Prodmtiis longispiiins, P. 

 nehrancensis, Chonelcs nrnala, C. mtsuloha, C. rariolala, C. saliiiaiiinnit, C. setUjc.nts (also Devouian), C. fischeri, etc., 

 ProdiicteUa newhen-i/i, besides the Permian Productus horrida. 



-Besides Paradoxides, there are such forms as the Cambrian llijdrocephaliix careiis, the Silurian DaJimaiia punctata, 

 Clieininis pleiirexantliemiiK, and Eiirijcare brccicaiidii, while the spiny species of Acidaspis seem to be more abundant in 

 the Devonian than iu the Silurian strata, but those which bear dorsal spines, such as Delphon foi-hesli aud Jrijis 

 annatus, are Devouian. 



■'Quite long spiues occur in the Cretaceous species of Crioceras and Aiici/loceras iiiatlteroiiiiiiiiiin of Europe, but 

 noue, so far as we are aware, iu earlier times. 



^See N. S. Shaler: Notes on Taxodiiini distichitm, Mem. M. C. Z., xvi, 1, 2, aud W. P. Wilson: The prodnctiou 

 of aerating organs on the roots of swamp and other plants, Proc. Acad. Nat. Sci. Phil., April 2, 1889, quoted in 

 Garden and Forest, .Jan. 1, 18.10. Shaler conjectures that the fuuction of the "knees" la iu some way connected 

 with the aeration of the sap. Mr. Wilson shows that " besides the cypress, other plants which habitually grow with 

 roots covered with water (the water gum, A''//i.sn silvalica, var. a(jiiaticn, Ariccnnin iiitida, aud Piiiits serotinit) develop 

 similar root processes ; aud what is still more suggestive, Jlr. Wilson has induced plants of In<lian corn to send roots 

 aljove the surface of the soil by keeping it continually saturated with water." It is to be observed that the aerial 

 roots of the latter develop iu a single generation. 



S. Mis. 50 2 



