478 TRANSACTIONS OF THE 



ics, after all our boasting, we of the nineteenth century look up 

 to the Pyramids with awe and stupidity, and ignorantly wonder 

 by what means they were built. No doubt the thing might be 

 easily done again if we only knew how. 



I am told by military surgeons that bayonet wounds very rarely 

 occur in their practice. But few men are bold enough to face a 

 naked point of steel. At the point of the bayonet England is 

 carrying her empire round the world, the stability of which is 

 based on the physiological law that inferior races of men cannot 

 stand the charge of this instrument. Yet Leonidas and the four 

 hundred, though "a forlorn hope" stood the charge of the bayonet 

 and fell to the last man rather than fly. Did the six hundred at 

 Balaklava do any thing more; or is th^re an instance in modern 

 history of greater bravery in as many men 1 



If then the variations of form in animals and plants are confined 

 within certain narrow limits and were so confined not only during 

 historic periods, but even during the ancient geologic epochs, 

 maintaining certain clearly defined types and preventing that suc- 

 cessive mixing 'of races that would have ended in the production 

 of a common form, it becomes a matter of exceeding interest to 

 enquire on what circumstances the limited variations that do take 

 place depend — to what extent they may go and within what 

 bounds they occur. 



CAUSES OF VARIATION IN ANIMALS. 



It is a well known fact that the muscles of men and animals, if 

 well supplied wfth nutriment, grow more powerful by exercise. 

 To produce this effect the exercise must be repeated at short in- 

 tervals, and must be sufficient to tire without exhausting, while 

 the food must be of the proper quality, and sufficient in quantity. 

 If the exercise be carried too far for the food, diminution instead 

 of augmentation of the bulk of the muscle results. The same is 

 true of the other organs of animals, and in this one law carried 

 out we shall find the origin of varieties in animals and plants. 



Liebig has shown indisputably, that two different parts of the 

 food of animals are used for two different purposes, viz : the 

 nitrogenized parts, for the nutrition of muscles, and consequently 

 for the production of motion while the purely carbonaceous parts 



