MEMOIES OF THE ^^-ATIOXAL ACADEMY OF SCIENCES. 
17 
'widespread change in the environment, involving a drying up of the soil, much of it alkaline, the 
direct iutlueuce on plant life must have been profound, as regards their protective defenses, and 
after spines 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 eases, have developed spines in response to a change of environ- 
ment. If we glance over the epochs of paleontological history we shall see tliat at certain periods 
trilobites, brachiopods, ammonites, and perhaps other groups showed a tendency to become tuber- 
cuhited, 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 x>erhaps of 
climate and soil. Among the brachio])ods, more spiny species occur in the Carboniferous period 
than in the earlier Paleozoic times.' Among the trilobites, although in Paradoxides and in other 
genera the geme 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 spiny 
forms of ammonites as in the uncoiled CretaceoUvS 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 \itality of the type was apparently declining. Geddes claims that the spines 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 foorl, it is 
not improbable that tlie appearance of such highly or grotesquely ornamented forms as certain 
later brachiopods, trilobites, and ammonites was the result of a change in their environment during 
a period when there were more widesiiread 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 roots, ^ or if, like epiphytes, when growing in mid-air, they throw 
out descending aerial roots, I liave thought it not improbable that tubercles, humps, or spines may 
have in the first place been developed in a few generations, as the result of some change in 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 and flowers probably began to appear, 
I have always 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 tlie larvm and the moths differ structurally far more than the 
genera of Geometrids and of Noctuidm, and the number of species is less. The two latter families 
probably arose.fi’om the great sx^eciaUzatioii of type iu Tertiary times; while evidently the great 
‘Although there are spiny brachiopods iu the Silurian, they become more common in the Dovoninn (e. g., Atnjpa 
hysirix, Chonetes scihila, C. coronataf C. vinricaia, Prodaclella /tirswfa, P. Uyatriciila^ P. 7'nri8pina, and Strophacosia 
producioides), and are apparently more numerous in the Carboniferous formation (o. g., Prodactiia hmgiapinnay P. 
nehrascensis, Choneien ornaiay C. mesoloha, C. variolatay C. salmaniami, C. aetlgcnis (also Dorouian), C. fiacheriy etc., 
ProdacteUa neivherryiy besides the Permian Productm horrida. 
^ Besides Paradoxides, tiiere are such forms as the Cambrian Uydrocephaluft caremy the Silurian Dalmania pnnclala, 
Cheintrus pleurcxantkemna, and Eurycare brevicauda, while the spiny species of Acidaspis seem to he more abundant in 
the Devonian than iu the Silurian strata, but those which hear dorsal spines, such as Deiphon forbeali and Jrges 
armatus, are Devonian. 
^ Quite long spines occur iu the Cretaceous species of Criocoras and A ncyloceraa maiheronianam of Europe, but 
none, so far as wo are aware, in earlier times. 
■•See N. S. Shaler: Notes on rAXodiam distichum, Mem. M. C. Z,, xvi, 1, 2, and W. P. Wilson: The production 
of aerating organs on the roots of swamp and other plants, Proc. Acad. Nat. Sci. Phil., April 2, 1889, quoted in 
Oarden and Forest, Jan. 1, 1890. Shaler conjectures that the function of the ‘‘knees’' is iu some way connected 
with theaoration of the sap. Mr. Wilson shows that “ besides the cypress, other plants which habitually grow with 
roots covered with water (the water gum, ^ysm ailvaticaf var. aqttalicay Avicennia vitidaj and Phius seroiina) develop 
similar root processes; and what is still more suggestive, Mr. Wilson has induced plants of Indian corn to send roots 
above 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 in a single generation. 
S, Mis. 50 2 
