1890 .] 
505 
[Packard. 
It is now assumed by some naturalists that the thorns, spines, 
and prickles of Cacti and other plants growing in desert or dry and 
sterile places are due either to defective nutrition, or to “ebbing vi- 
tality” (Geddes) ; or by others, as Mr. Wallace, to the stimulus re- 
sulting from the occasional attacks or visits of animals, especially 
mammals. It should be borne in mind that the great deserts of the 
globe are of quite recent formation, being the result of the desicca- 
tion of interior areas of the continents, late in the Quaternary epoch, 
succeeding the time of river-terraces. Owing to this widespread 
change in the environment, involving a drying up of the soil, much 
of it alkaline, the direct influence on plant-life must have been pro- 
found, as regards their protective defences, 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 cases, have developed spines in re- 
sponse to a change of environment. If we glance over the epochs 
of palaeontological history we shall see that at certain periods, tril- 
obites, brachiopods, ammonites, and perhaps other groups, showed 
a tendency to become tuberculated, 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 perhaps of climate and soil. Among the Brachiopods, 
more spiny species occur in the Carboniferous period than in the 
earlier Palaeozoic times . 1 Among the trilobites, although in Para- 
doxides and in other genera, the genae 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 . 2 There 
of the carapace of the embryo and larval Decapod Crustacea, and also in insects. For 
example, note the change in form and partial atrophy of the two hinder thoracic somites 
of some beetles, as compared with the large prothorax, due probably to the more or less 
continual pressure exerted by the folded elytra and wings. 
1 Although there are spiny Brachiopods in the Silurian, they become more common 
in the Devonian (e. g., Atrypa hystrix , Chonetes scitula, C. coronata, C. muricata, Pro- 
ductella hirsuta, P. hystricula, P. rarispina and Strophacosia productoides) , and are 
apparently more numerous in the Carboniferous formation (e. g., Productus longispinus 
P. nebrascensis, Chonetes ornata, C. mesoloba, C. variolata, C. salmaniana, C. setigerus 
(.also Devonian), C.fischeri, etc. Productella newberryi, besides the Permian Productus 
horrida. 
2 Besides Paradoxides, there are such forms as the Cambrian Hydrocephalus carens, 
the Silurian Dalmania punctata, Cheirurus pleurexanthemus and Eurycare brevicauda, 
while the spiny species of Acidaspis seem to be more abundant in the Devonian than 
in the Silurian strata, but those which bear dorsal spines, such as Deiphon forbesii and 
Arges armatus, are Devonian. 
