30 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



Ernst Haeckel (1834-1919) was one of the earliest and most 

 influential followers of Darwin in Germany. In his Generelle Mor- 

 phologie, published in 1866, seven years after the Origin of Species 

 first appeared, he applied the doctrine of evolution, and especially 

 the theory of natural selection, to the whole field of vertebrate mor- 

 phology. Beyond question Haeckel overapplied the theory and in a 

 sense weakened its influence by his rather uncritical use of materials. 

 His writings have been translated into most languages and ''are 

 popularly believed to represent the best scientific thought on the 

 matter. " Biologists today, however, are apt to look askance at 

 Haeckel's works and to consider that they did more harm than good 

 to Darwinism. 



August Weismann (1834-1914) was the first really original 

 evolutionist after Darwin. Like other thinkers of his time, he realized 

 that further progress in the knowledge of the causal basis of evolution 

 lay in further investigation of the causes of variation and the physical 

 basis of heredity. Weismann has been classed as a neo-Darwinian 

 because he was a strong advocate of some form of selection, but his 

 "selection" was not the selection of Darwin. Realizing that the 

 greatest weakness of the natural-selection theory lay in its inadequacy 

 as an originator of variations, he proposed the "germinal-selection" 

 theory. He contended that all heritable variations have their origin 

 in the germ cell, and therefore that a new type of organism arises only 

 from a changed type of germ cell. The germinal-selection theory 

 stands out in striking contrast with Darwin's "pangenesis" theory. 

 The former is centrifugal, the latter centripetal. "Determiners" of 

 new characters, according to Weismann, arise in the germ plasm and 

 work outward to all parts of the developing body; while the "gem- 

 mules," Darwin's equivalent of determiners, originate in the body 

 tissues and are carried to the germ cells in each generation. Accord- 

 ing to Weismann, there is a struggle among the determiners for the 

 available food and favorable positions in the germ cell, and those that 

 receive the most food and the best positions gain an initial advantage, 

 so that they are able to initiate the development of larger or more 

 perfectly adapted organs. The descendants through cell division of 

 these favored determiners are in a position to compete with other 

 determiners on a more favorable footing in each succeeding generation, 

 so that the character represented by them steadily increases in a linear 

 or definitely directed fashion until it reaches the state of complete 

 adaptation or fitness. Such a character may even continue its direct 

 line of advance beyond the point of maximum fitness and result in 



