598 VICTOR E. SHELFORD 
In nature this law applies both geographically and locally. As 
applied to animals it includes both food and material for abode. 
The presence, absence and success of a species is determined by 
the necessary material which is absent or present in minimal 
quantity. 
c. Law of toleration of physv:al factors. We have noted (p. 
581) in the case of the tiger beetles, that for the egg-laying to take 
place the surrounding temperature and light must both be suit- 
able, the soil must be moist, probably also warm, and must 
satisfy the ovipositor tests with respect to several factors. Egg- 
laying, the positive reaction, is then probably a response to several 
factors. Furthermore, after the eggs are laid, the conditions 
favorable for egg-laying must continue for about two weeks if 
the eggs are to hatch and the larvae reach the surface of the ground. 
The success of reproduction depends, then, upon the qualitative 
and quantitative completeness of the complex of conditions. 
The negative reaction, on the other hand, appears to be different. 
The absence of eggs, the number of failures to lay and therefore 
the number of eggs laid in any situation can be controlled by qual- 
itative or quantitative deficiency or excess with respect to any 
one of several factors. The presence, absence, or number of eggs 
laid is, then, determinable by a single factor, according as it is 
near the optimum or near either the maximum or minimum tol- 
erated by the species. It is, however, not necessary that a single 
factor deviate; the effect is similar or more pronounced if several 
deviate. 
In nature the presence or absence, or success of a species or 
group of species, its numbers and sometimes its size, etc., are 
largely determined by the degree of deviation of a factor or 
factors from the range of optimum of the species or group of 
species. The cause of the deviation in the factor or factors is 
not of importance. For example, in the case of a soil inhabi- 
ting species such as Cicindela tranquebarica, to which consider- 
able moisture is necessary, the cause of the deficiency in one case 
may be climatic deficiency in rainfall, in another a rapid drainage 
due to steep slope and porosity of soil. The former is what we 
have called a climatic (geographic) condition and the latter a 
