Packard.] 
506 
[Feb. 19, 
are no such spiny forms of Ammonites as in the uncoiled Creta- 
ceous Crioceras , 1 etc. 
These types, as is well known, had their period of rise, culmina- 
tion and decline, or extinction, and the more spiny, highly orna- 
mented, abnormal, bizarre forms appeared at or about the time when 
the vitality of the type was apparently declining. G-eddes 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 food, it is not improbable that the appearance of such highly or 
grotesquely ornamented forms as certain later brachiopods, trilo- 
bites 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 oc- 
curred. 
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 , 2 
or if, like epiphytes, when growing in mid air, they throw out de- 
scending aerial roots, I have thought it not improbable that tuber- 
cles, 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 Pa- 
laeozoic or the early part of the Mesozoic age, the time when decid- 
uous trees and flowers probably began to appear. 
I have always regarded the Bombyces, or the superfamily of silk- 
worm moths, as a very ancient one, which has lost many forms by 
geological extinction. We thus account for the many gaps be- 
tween the genera. Both the larvae and the moths differ structu- 
1 Quite long spines occur in the Cretaceous species of Crioceras and Ancyloceras 
matheronianum of Europe, but none, so far as we are aware, in earlier times. 
2 See N. S. Shaler: Notes on Taxodium 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 “Garden and Forest,” Jan. 1, 1890. 
Shaler conjectures that the function of the “ knees ” is in 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, Nyssa silvatica, var. 
aquatica, Avicennia nitida and Pinus serotina ) 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. 
