96 



SCIENCE 



[N. S. Vol. XLI. No. 1046 



latest statement of his views, Bateson argues 

 substantially as follows: Variations may be 

 large or small. Those which are small are 

 either not inherited or are of no consequence, 

 being " slight differences that systematists 

 would disregard." But large differences can 

 not arise " by accumulation of small differ- 

 ences." Hence only large differences have 

 evolutionary significance. In his own words :^ 



We have done with the idea that Darwin came 

 latterly to favor, that large difEerences can arise 

 by accumulation of small difEerences. Such small 

 difEerences are often mere ephemeral efEeets of 

 conditions of life, and as such are not transmiss- 

 ible; but even small difEerences, when truly ge- 

 netic, are factorial like the larger ones, and there 

 is not the slightest reason for supposing that they 

 are capable of summation. 



Whether we may properly regard small differ- 

 ences as capable of " summation " depends 

 upon what we mean by summation. Phillips 

 and I^ have shown that in the case of piebald 

 rats the areas of white fur characterizing the 

 race may be either increased or decreased at 

 will and that the change takes place gradually, 

 progressing steadily generation after genera- 

 tion and far transgressing the original limits 

 of variation. The same is undoubtedly true 

 of similar variegated patterns which mendelize 

 among both animals and plants. Small differ- 

 ences which have arisen spontaneously have 

 certainly been aggregated in this case. But 

 crossing of the modified races shows that the 

 aggregated changes have not been summated 

 to such an extent that they constitute a single 

 Mendelian factor, except in one case, where it 

 seems quite possible that something of this 

 sort has occurred. I am by no means ready to 

 regard summation out of the question, 

 whether by that we mean mere aggregation or 

 fusion into a new Mendelian unit. 



Bateson has further expressed the view that 

 evolution has occurred largely, if not exclu- 

 sively, by loss of Mendelian factors resulting 

 in striking variations that breed true from 

 their first appearance and thus render the 



5 L. c, p. 285. 



« Publ. No. 195, Carnegie Institution of Wash- 

 ington, 1914. 



parent species dimorphic or polymorphic. That 

 many varieties of domesticated animals and 

 cultivated plants have originated in this way 

 will be admitted by any one who has studied 

 them genetically. Darwin himself pointed 

 out the importance of " sport " variation in 

 producing new varieties of animals and plants 

 under man's care and supervision and it is 

 known that similar variations occur in wild 

 species. But it is doubtful whether in a wild 

 species a sport originating in this way has 

 ever replaced the original form. Under do- 

 mestication it is only the constant interposi- 

 tion of man that keeps the favored sports alive. 

 Whether sport variation has had any part in 

 the evolution of species is accordingly very 

 doubtful. If we compare one wild species 

 with another, we commonly find existing be- 

 tween them not single striking differences 

 but numberless minute differences. Systemat- 

 ists usually name as diagnostic characters a 

 few of the more striking differences, ignor- 

 ing, as they are quite warranted in doing, all 

 minor ones, the enumeration of which is for 

 their purpose superfluous. But if one makes 

 an intensive study of related species he finds 

 that they differ in endless details of structure 

 and physiological behavior extending even to 

 differences in size of the constituent cells of 

 the body (Conklin), or of their parts (chromo- 

 somes, chromomeres, etc.). During recent 

 years, as the discrimination of species has be- 

 come more keen, it has talten on more and 

 more a quantitative expression. Series of 

 specimens are measured, and specific distinc- 

 tions are based on absolute and relative di- 

 mensions of parts, not on the " presence or 

 absence " of large striking features of organi- 

 zation. It is easy to dispose of the work of 

 the systematist by assuming that he does not 

 know his business, but is it wise to do so? 

 For other lines of evidence also indicate that 

 the differences between species are quantita- 

 tive and increase with genealogical divergence. 

 This for example is the conclusion reached 

 from such distinct methods of study as the 

 examination of the forms of hemoglobin crys- 

 tals in the blood of various species of animals 

 and of the precipitin reaction of the blood. 



