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more time in development, both in the egg and fi-om the im- 

 mature to the mature state, the same principle should hold good 

 among the Mammalia in the duration of time in the foetal and 

 immature state ; and this can very easily be shewn to be the case. 



The period of gestation is, in the Elephant, 20 to 21 months ; 

 the Giraffe, 14 months; the Dromedary, 12 months; the Mare, 

 1 1 months ; the Cow, 9 months ; the large Deer, 8 months ; the 

 Bear, 6 months ; the Sheep and Goat, 5 months ; the Sow, 4 

 months ; the Dog, Wolf, and Fox, 63 days ; the Cat, 5G days ; 

 the Hare, 30 days and upwards ; the Rabbit, 28 days ; the Squir- 

 rel and Eat about the same time ; and in the Mouse, the period 

 of gestation is a fortnight. 



The Implacental, or Marsupial animals, which produce their 

 young in an imperfect state, have shorter periods of gestation 

 than the Placental of similar size. 



The period in the Kangaroo is 39 days, in the Opossum but 26 

 days, and in the small Australian species a much shorter period. 



I will not trouble you with many illustrations of the ftict that 

 large animals require after their birth a longer time to attain 

 maturity than small ones. 



The " Right Whale " is believed to be full grown in its twenty- 

 fifth year. 



The Elephant is many years old before he is adult, and the same 

 may be said of the Hippopotamus and the several species of 

 Rhinoceros. The Horse and Ox are full grown at three to four 

 years of age, the Sheep during the second year of its life. Hares 

 and Rabbits are full grown at four to five months old, and Rats 

 and Mice in a still shorter period. 



I think the facts stated are sufficient to establish the truth of 

 the postulate I started with, that the higher an animal is de- 

 veloped the longer the time required for its development both in 

 the foetal and immature state. 



There appears to me to be another law, to which however I can 

 but very briefly advert, and that is, that the higher the develop- 

 ment and the larger the size, the less the number, both in species 

 and specimens, at present existing on the earth. 



I will illustrate my meaning by comparing the relative number 

 of species in three of the orders of Mammalia, which I have 

 selected because the affinities and position of the smaller orders 

 are neither well known nor their limits defined. 



The Quadrumania, confessedly the highest order, contains about 



