EXTERNAL INFLUENCES AS CAUSES OF VARIATION 



257 



Commenting on this table, its author observes in substance : 



1. That in general the optima, the minima, and the maxima 

 rise and fall together ; that is, a species with a high optimum 

 will also have a relatively high maximum and minimum. 



2. That species vary greatly ; so much so that the maximum 

 of one (B. phosphorescens) may be below the minimum of another 

 (B. thermophilus}. 



3. That the optimum for the radicle and the plumule may be 

 widely apart, as in the bean. 



4. That, in general, the optimum is in close relation to the 

 natural habitat of the species, as in B. phosphorescens that lives 

 in the moderate temperature of the sea, and in B. thermophilus 

 that lives in the high temperature of decaying manure. From 

 collateral evidence this must be ascribed to acclimatization. 



5. That of all the species noted, the bacteria have the greatest 

 range in optimum, showing that they are, as yet at least, less 

 fixed in their organization. 



6. That the minimum never falls below o C., the freezing 

 point of water, which is the minimum for vital activities. 



7. That the maximum temperature tends to be rather constant 

 with related species, and among flowering plants the range is but 

 9 (37-46). But 46 is a fatal temperature for most proto- 

 plasm, and 50 is the limit, showing how near the limit some 

 species have been pushed. The extraordinarily high temper- 

 atures of B. thermophilus must be regarded as an instance of 

 acclimatization, of which other striking examples are found in 

 hot springs. 1 



8. That the range from minimum to maximum varies with the 

 species. In this table the range is least for B. tuberculosis (12) 

 and greatest for B. phosphorescens (37). 



9. That the " wonderful adjustment " of critical temperatures 

 to the environment of the species is not to be regarded as 

 evidence of selection, but, as is elsewhere shown under " Accli- 

 matization," it is due to the modification wrought in the proto- 

 plasm by the temperature itself. 



1 To the above may be added the observation that the optimum lies nearer 

 the upper limit ; that is, the difference between the optimum and the maximum is 

 less than the difference between the optimum and the minimum. 



