RODENTIA 247 



ance with the general former practice, though there is a strong re- 

 cent tendency toward placing them by themselves in a distinct order. 

 In North America alone 77 genera were recognized in 1910, 44 of 

 which, with 750 species and varieties, occur north of Mexico. 1 Many 

 have since been described or recorded. 



Not only are there more genera and species than in any other order, 

 but probably there are more individuals of rodents than there are of all 

 other orders combined. Many of the rodent groups are very prolific 

 breeders. For example, field mice have from 6 to 8 young to* the 

 litter, with a gestation period of 21 days; one female in captivity 

 had 17 litters in i year; another had 13 litters before she was a year 

 old ; if all the young had lived and bred at the same rate the progeny 

 would have numbered 1,000,000 in a year. 2 In these extreme cases 

 fecundity was probably overstimulated by shelter, plenty of food and 

 other favorable conditions provided in captivity, but the normal fer- 

 tility in a state of nature is enormous. Were the death rate not also 

 very high they would soon overrun the earth and destroy all vege- 

 tation. Fortunately after they increase in numbers to a certain point, 

 owing to temporary favorable conditions of weather, food, cover and 

 so on, the mortality usually greatly increases, owing to lessening of 

 food, crowded conditions, epidemic diseases, etc., so that their num- 

 bers are again reduced to or below the normal abundance. 3 



For the house rat, assuming from 3 to 6 litters per annum, 

 with an average of 10 young to each litter, various estimates of 

 possible results, if all lived and bred, have been made. Lantz made 

 the number 20,155,392 in 3 years, on a 3-litter per annum 

 basis, or, by a later computation, on a 6-litter basis, 359,709,482. 

 Rucker says 940,369,969,152 in 5 years, while Fischer says 

 48,319,698,843,030,344,720 in 10 years. True, these figures are obvi- 

 ously impossible, based as they are upon the assumption that all lived 

 and bred. 4 However, such computations are valuable for the purpose 



1 Lantz, Rodent pests of the farm, Farmers' Bull., No. 36, 1910. 



2 Bailey, Breeding, feeding and other life habits of meadow mice (Microtus), 

 Journ. Agric. Research, xxvii, 523-535, 1924. 



3 Howell, Periodic fluctuations in the numbers of small mammals, Journ. Mam- 

 malogy, rv, 149-155, 1923- 



4 Silver, The reproduction potential of rats, Journ. Mammalogy, v, 66-67, 1924; 

 citing various publications. See also Eaton and Stirrett, Reproduction rate in wild 

 rats, Science, LXVII, 555-556, 1928. Donaldson, Control of rat population, Science, 

 March 20, 1925. Forbush, Rats and rat riddance, Mass. Board Agric., Econ. Biol., 

 Bull. No. i, pp. 11-12, 1915. Rodwell, The rat, pp. 167-168, 1858. Lantz, The brown 

 rat in the United States, U. S. Biol. Surv. Bull. No. 33, p. 16, 1909. The rat and 

 its relation to public health, U. S. Public Health and Marine Hospital Service, p. 

 153, 



