INTRODUCTION 13 



Spencer, Cope, Hyatt, Osborn, Semper, Claus, Roux and others), which ranges 

 itself more and more on the side of Lamarckian ideas, and ascribes to the use 

 and disuse of organs, and to external conditions, a very considerable influence 

 in effecting the transformation of organic forms. AVhile, on the one hand. 

 Semper, Locard and Clessin undertake to prove the direct action of environ- 

 ment on mollusks in a number of instances ; 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 " convergence," which are 

 in nowise related to one another by inheritance. The analogous swimming- 

 organs of fishes, ichthyosaurians and whales, or the analogoiis 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 Pferosauria, or for the 

 spindle-shaped body characteristic of most rapid-swimming fishes, reptiles and 

 aquatic mammals, oi; for the similar form of jaw possessed by marsupials and 

 various orders of Placentalia. These are all instances of [)arallelism, 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 the line of progressive modification in fossil genera as exemplified by 

 numerous series of intermediate forms. 



In sharp contrast to all these opinions is the " mutation theory " of de 

 Vries. The latter attempts to show that new species of plants are formed 

 by what he calls mutations. It should be noted that this term is used in a 

 different sense from the same word as mentioned on p. 10, being equivalent to 

 saltatioii, used previously for the same thing. Mutations, in de Vries's sense, 

 are more or less strongly marked deviations from the normal type, appearing 

 rather suddenly ; and de Vries claims that only these are capable of being 

 bred true by pedigree-culture, and that they alone lead to the origin of new 

 species. What he actually did was to demonstrate that it is possible, by 

 pedigree-culture, to produce true breeding forms (species) out of mutations, 

 but he failed to see that the essential factor in this process is not the quality 

 of the material he worked with (i.e. the mutations), but that it is the pedigree- 

 culture, and that this corresponds to the well-known factors of selection and 

 isolation. 



The latter principle, originally introduced, as has already been stated, by 

 Moritz Wagner, has recently been put forward by other writers (Baur, 

 Ortmann, Gulick, etc.) as a factor which causes the differentiation of one 

 species into several co-existing species (" process of speciation," 0. F. Cook). 

 While it is admitted that the Lamarckian factors of variation and inheritance 

 and the Darwinian factor of natural selection, are real and actual, it is 



