INTRODUCTION 13 
and disuse of organs, and to external conditions, a very considerable influence 
in effecting the transmutation of organic forms. While, on the one hand, 
Semper, Locard, and Clessin undertake to prove the direct action of environ- 
ment on mollusks in a number of instances, and Schmankewitz confidently 
asserts that the transformations in Artemia are induced by changes in the 
saltness of the water; on the other hand, Cope, Osborn, Roux, and others, 
emphasise the effect of use and disuse, and abundance or scantiness of food- 
supply. Adequate nourishment and exercise increase the development of a 
given organ, while physical conditions determine its form. Since like causes 
produce “hike effects in the animate as well as in the inanimate world, it is 
obvious that similar organs must be developed in a variety of plant and animal 
forms wherever they are subjected to similar external conditions, and especially 
to the same physical agencies. A convenient explanation is thus found for the 
phenomena of parallelism, or recurring “‘ common types of structure,” which are 
in nowise related to one another by inheritance. The analogous swimming- 
organs of fishes, ichthyosaurians, and whales, or the analogous limb-structure 
in long-legged ruminants, the horse, elephant, and carnivora, are due to 
adaptation to external conditions and to use; the same explanation also 
accounts for the like form of sternum in bats, birds, and Plerosauria, or for the 
spindle-shaped body characteristic of most rapid-swimming fishes, reptiles, and 
aquatic mammals, or for the similar form of jaw possessed by marsupials and 
various orders of Placentalia. These are all instances of parallelism, in which 
it often happens that two fundamentally different forms acquire the same 
outward shape, or become provided with similar or analogous organs. Kineto- 
genesis, or the process of a gradual transformation of parts, ecneclly parts 
belonging to the internal deleion, skull, and limbs, is very ingeniously 
interpreted by Cope as having been accomplished in mammals through the 
agency of mechanical conditions, use, and food. The same author has also 
traced out the different stages of development in fossil genera as exemplified 
by numerous series of intermediate forms. 
Life-Period and Extinction of Species.— Observation shows that. 
different organisms are by no means equally susceptible to impulses received 
from the outer world. Many fossil genera remain almost wholly unchanged 
throughout a number of formations (foraminifera, Cidaris, Nautilus, Lingula, 
Terebratula, Insectivora), and hence may be designated as persistent or conservative 
types, in contradistinction to variable types. The latter pass through rapid changes 
at the beginning of their career, develop a great variety of forms, and send out 
branches and offshoots in all directions up to a certain point ; they may then 
die out after a comparatively short period of ascendency (Nummulites, Gr aptolites, 
Cystids, Blastoids, Tetracoralla, Palechinoidea, Trilobitae, Rudistae, Ichthyosauria, 
Pterosauria, Dinosauria, Amblypoda, Toxodontia, etc.), or in some cases may even 
continue on to the present day with undiminished vitality (Spatangidae, 
Clypeastridae, many land and fresh-water mollusks, crabs, lizards, snakes, 
ruminants, apes). Not infrequently types that were primitively variable pass 
over gradually into persistent ; their power of adaptation dwindles, they grow 
less plastic, become incapable of sending off new varieties, species, or genera, 
and as the less vigorous of their number become worsted one after another , they 
finally stand out ‘like curious, isolated relics of antiquity (Pentacrinus, Hatteria, 
Tapirus, Equus, ete.) in the midst of subsequently remodelled surroundings. 
A one-sided development in a certain direction, excessive size, abnormal 
