290 THE INDUCTIONS OF BIOLOGY. 



Bearing in mind that the slightly-different orders of phy- 

 siological units which an organism inherits from its parents, 

 are subject to the same set of forces ; and that when the 

 organism is fully developed, this set of forces, becoming con- 

 stant, tends slowly to re-mould the two orders of units into 

 the same form ; we see how it happens that self-fertilization 

 becomes impossible in the higher organisms, while it remains 

 possible in the lower organisms. In long-lived creatures that 

 have tolerably-definite limits of growth, this assimilation of 

 the somewhat-unlike physiological units, is liable to go on to 

 an appreciable extent ; whereas in organisms which do not 

 continuously subject their component units to constant forces, 

 there will be much less of this assimilation. And where the 

 assimilation is not considerable, the segregation of mixed 

 units, may cause the sperm-cells and germ-cells developed in 

 the same individual, to be sufficiently different to produce, by 

 their union, fertile germs ; and several generations of self- 

 fertilizing descendants may succeed one another, before the 

 two orders of units have had their unlikenesses so far diminish- 

 ed, that they will no longer do this. The same principles 

 explain for us the variable results of union between nearly- 

 related organisms. According to the contrasts among the 

 physiological units they inherit from parents and ancestors ; 

 according to the unlike proportions of the contrasted units 

 which they severally inherit ; and according to the degrees 

 of segregation of such units in different sperm-cells and 

 germ-cells ; it may happen that two kindred individuals will 

 produce the ordinary number of offspring, or will produce 

 none ; or will at one time be fertile and at another not ; or 

 will at one time have offspring of tolerable strength, and at 

 another time feeble offspring. 



To the like causes are also ascribable the phenomena of 

 Variation. These are unobtrusive while the tolerably-uni- 

 form conditions of a species maintain tolerable uniformity 

 among the physiological units of its members ; but they 

 become obtrusive when differences of conditions, entailing 



