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 like 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 Pterosauria, 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, especially parts 

 belonging to the internal skeleton, 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 off-shoots in all directions up to a certain point ; they may then 

 die out after a comparatively short period of ascendency (Nummidites, Graptolites, 

 Cystids, Blastoids, Tetracoralla, Palechinoidea, Trilobitae, Eudistae, 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 pas& 

 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, etc.) in the midst of subsequently remodelled surroundings. 

 A one-sided development in a certain direction, excessive size, abnormal 



