November 29, 1907] 



SCIENCE 



755 



types of organic variation will be revealed, 

 which are highly suggestive. 



That each organic form depends on a special 

 environment, and is able to exist only under 

 a certain set of conditions, is generally 

 knovsTi," but a specific environment is by no 

 means a firmly fixed, unalterable and un- 

 changeable factor, even if we are justified in 

 saying that in a particular case the environ- 

 ment has remained or remains the same. 

 There are, indeed, certain major features in 

 each set of environmental conditions, which 

 remain unaltered, but there are always others 

 which may and do vary. 



To make this more clear, let us take a single 

 factor as an example, for instance, climate. 

 Each organic form depends on a cer- 

 tain climate, but climate is not a fixed, per- 

 manent and uniform factor, even within the 

 tropics. What we call climate is an average 

 condition resulting from positive and negative 

 deviations of a number of factors (tempera- 

 ture, precipitation, etc.) from a normal value. 

 Now we all know that these deviations from 

 the average again are not the same in differ- 

 ent periods : we talk of daily, monthly, sea- 

 sonal changes, and even the average conditions 

 of different years are not identical, so that we 

 observe periods of climatic variations extend- 

 ing over a number of years. , 



The same is true of any other environ- 

 mental condition, and if we keep in mind the 

 great complexity of factors which enter into 

 the concept of " environment," which will be 

 best under-stood by those who have paid atten- 

 tion to the modern studies in ecology, we shall 

 be able to correctly estimate the value of this 

 idea, that the environment, the ecological con- 

 ditions, under which a given species lives7 is 

 not a fixed set of unchangeable features. 

 There is hardly a factor which is constant, but 

 it generally goes at certain times beyond a 

 certain average or optimum in one or another 

 direction. 



These deviations from the normal state we 

 may call by the name of " fluctuations," a term 

 which is familiar. The chief feature of fluc- 

 tuation is that, when there has been, for a cer- 



' See Brooka, in Proc. Amer. Philos. Soc, 45, 

 1B06, p. 75. 



tain time, a deviation from the normal in one 

 direction, it is compensated, more or less, at 

 another time, by a deviation in another direc- 

 tion. Sticking to our example of the climate: 

 the darkness and coolness of the night are 

 compensated by the light and the heat of the 

 day; the sun shines in the morning from the 

 east, this is compensated by the position of the 

 sun in the evening; cool and damp days alter- 

 nate with hot and dry days ; the seasonal cycle 

 of the year is characterized by opposite condi- 

 tions prevailing at different seasons. Further, 

 there may be a series of years of unusual dry- 

 ness, of excessive heat, which is counter- 

 balanced by a series of years of the opposite 

 character, and so forth. 



Yet we know that variations of environ- 

 ment are not always of this character, fluctua- 

 ting around an average, which remains more 

 or less constant. We have ample evidence of 

 variation, which tends to change the average 

 condition in a certain direction, and to change 

 it permanently. We know that the climate 

 of a country changes, that it becomes, in the 

 course of time, a different one from what it 

 used to be. I hardly need to mention ex- 

 amples, since everybody knows what I am re- 

 ferring to: yet the change of environment 

 brought about by the advent of the Glacial 

 period may be quoted, and also the change 

 caused in the environmental features of this 

 continent by the immigration of the white 

 man. 



To one thing, however, I want to call special 

 attention. There is no sharp line to be drawn 

 between variation fluctuating around a certain 

 average, and a permanent change of the latter. 

 As we have seen, the average may change for 

 a short time in one direction, and may go back 

 subsequently in the other direction ; but gradu- 

 ally the change of the average in one direction 

 may begin to prevail, it may not entirely be 

 compensated by the opposite movement, and 

 finally the latter may be suppressed altogether, 

 so that only the movement in one direction 

 remains, which then may end in the establish- 

 ment of a new set of environmental condi- 

 tions. This change we may call by the name 

 of "mutation," and the use of this term in 

 this sense will be justified later on. 



