148 Evolution of Animal Life. 



istics with the features of ancestral resemblance. The 

 facts, both of variation and of heredity, are known but im- 

 perfectly; and their laws have not bt>cn discovered. 



L. The fact of multiplication, namely, that even the 

 slowest-breeding species of plants or animals, if permitted 

 to increase at its normal rate, would have crowded the globe 

 long ago, as is shown not only by theoretical calculations, 

 according to the rules for the summation of geometrical 

 series, but also by well-known and recent instances, in which 

 single species, imported into regions new to them, have 

 spread with astonishing rapidity, sometimes to the extinc- 

 tion of native species. The Canada thistle and the Norway 

 rat in this country, the wild horses of Mexico, the English 

 grasses in Australia and the rabbits in Tasmania, are famil- 

 iar and striking examples. 



M. The fact of population, namely, that this rapid nor- 

 mal increase does not, in general, take place, but, on the 

 contrary, the numbers of each species, in the alDsence of de- 

 cisive changes introduced by nature or man, remain com- 

 paratively stationary. Occasional decimation, as, for in- 

 stance, by exceptional weather or famine, is quickly made 

 good by the increase of the species again to its normal pro- 

 portion. The disturbance of this proportion by man is oft- 

 en followed by the rapid increase of some other species, 

 previously held in check by the one he has destroyed or 

 driven away. 



N. The fact of the effective life-period, as concerned in 

 this inquiry, namely, that animals have fulfilled the func- 

 tion of life when they have been born, grown to maturity, 

 produced and (in some cases) nurtured their normal number 

 of offspring. Until these functions are completed, death 

 is premature; afterwards, it is natural, and, so far as this 

 inquiry is concerned, relatively insignificant. N, it will be 

 seen, includes the sphere of "sexual selection." 



0. The fact of competition and struggle among individ- 

 uals and species and against the forces of nature, for food, 

 strength, shelter, victory over enemies or escape from them, 

 and for the production of offspring, etc., in short, a struggle 

 for effective life, as defined under N. 



P. The fact of the *' premature" death of the majority 

 (generally the vast majority) of each generation. 



Q. The production, by selection and close breeding, of 



