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 
