250 ESSENTIALS OF BIOLOGY 



multiple values ; that one range of values, v ^ e/is more probable 

 than any other one ; that there are other similar ranges of values 

 that are less probable and that the further removed from the mean 

 is any range of values the less probable it is. This is exactly 

 what we find in the frequency distribution of any one organic 

 character in a number of organisms. We tend to think of some 

 privileged value of the character — a value that " ought to " exist, 

 but what we really observe in nature are multiple values that are 

 more or less probable. 



856. Organic Fluctuations and the Environment. Even 

 in an irreducible category of organisms there are such fluctuations. 

 We may, for instance, establish a " pure race " of bean plants, 

 all of them being derived from (or are the progeny of) a single 

 bean and being perpetuated asexually. Even in a single pod borne 

 by such a plant the individual beans will differ in weight, and we 

 are inclined to see that these individual variations are due to the 

 environmental conditions : slight differences in position, in 

 conditions of nutrition, etc. But it is easy to see that the environ- 

 mental influences only condition the range of the fluctuations. 

 Thus we may incubate a number of eggs of, say, a pelagic fish and 

 observe that the mean period of incubation (that is, the time that 

 elapses between fertilization and hatching) is so many hours but 

 that this period varies considerably in individual eggs. If now 

 we change the temperature of the sea-water in which the eggs 

 incubate we shall see that the mean period of incubation is changed 

 (being increased for a reduction of temperature and vice versa). 

 But there will still be a somewhat similar range of variation in 

 the times at which the individual egg hatches out in the changed 

 environment and we conclude that the existence of fluctuations 

 is independent of these latter changes — which only influence the 

 position of the central point about which the variable character 

 fluctuates. 



What we observe in such cases as the growth of the beans, 

 or the embryogenies of the fish eggs are instances of a repetitional 

 developmental process — something roughly analogous to the series 

 of operations by which a minting machine strikes out coins intended 

 to be of precisely the same forms and weights (though it is only 

 by analogy that we think of the " intention " of the developmental 

 process, or organization). But it is clear that the products of 

 the minting machine are not precisely alike, because in the process 



