ORIGIN OF THE SPECIFIC TYPE 319 



struggle for existence decides which shall survive and which shall be 

 eliminated. For the mutations themselves occur in no particular 

 direction ; they are sometimes advantageous, sometimes indifferent, 

 sometimes even injurious (for instance, when one sex is left out), and 

 so it is always only a fraction of the mutations, often only a few, 

 which prove themselves capable of permanent existence. Thus 

 ' species do not arise through the struggle for existence, l)ut they 

 are eliminated by it ' (p. 150) ; natural selection does nothing more 

 than weed out wdiat is unfit for existence, it does not exercise any 

 selective, in the sense of directive, influence on the survivors. A dif- 

 ference in the nature of variations was previously maintained by 

 the American paleontologist Scott, though for different reasons and 

 also with a different meaning. He believed that variations in 

 a definite direction w^ere necessary to explain the direct course of 

 development which many animal groups, such as the horses and the 

 ruminants, have actually followed, and which he thought could not be 

 ascriljed to cumulative adaptation to the conditions of life. Thei 

 ' mutations ' of De Tries are not distinguished from the ' fluctuating ' 1 

 variations Ijy following a definite direction, but in that they are 

 strictly heritable, that they ' breed true.' It is true that ' fluctuating ' ■ 

 individual diflerences are also transmissible, and can be increased by 

 artificial selection, but they lack one thing that would make them 

 component parts of a natural species, namely, constancy ; they do 

 not breed true, and are therefore never independent of selection, but 

 require to be continually selected out afresh in order that they may 

 be kept pure. They form ' breeds,' not species, and if left to them- 

 selves they soon revert to the characters of the parent species, as is 

 well known of the numerous ' eniiobled races ' among our cereals. 

 De Vries therefore denies absolutely that a new species could be 

 developed by natural selection from ' fluctuating ' variations, and not 

 alone because there is no constancy of character, l»ut also because the 

 capacity of the character for being increased is very limited. Usually 

 nothing more can be achieved than doubling of the original character, 

 and then progress becomes more difficult and finally ceases altogether. 

 These are incisive conclusions, based upon an imposing array of 

 weighty facts. I readily admit that I have rarely read a scientific 

 book with as much interest as De Vries's MutationdTieorie. Never- 

 theless I believe that one might be carried away too far by De Vries, 

 for he obviously overestimates the value of his facts, interesting and 

 important as these undoubtedly are, and under the influence of what 

 is new he overlooks what lies before him — the other aspect of the 

 transmutation of snecies, to which the attention of most observers 



