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Ch. XXXVI.] 



PAETICULAE PAETS ONLY ALTEEED. 



297 



The nature of the food given to the caterpillar influences to 

 a certain extent the character of the breed. Great care is 

 taken in India and Europe in the selection of the eggs of 

 moths the caterpillars of which have produced the best 

 cocoons. The silk is usually yellow, but sometimes white, 

 and, by careful selection, in the course of sixty-five genera- 

 tions the proportion of yellow cocoons in France was greatly 



reduced. 



abdominal 



Quatref; 



always white. 



while the feet of those which give yellow cocoons are in- 

 variably yellow, and there is a corresponding difference in the 



tint of the eggs. 



Man causes particular parts of an animal or plant to vary 



vjJiile other parts may continue unaltered. — The possibility of 



obtaining particular breeds and fixed varieties of animals and 



plants depends on the fact that variations occur in almost 



any required direction if a vast number of individuals are 



produced. It is also found that one form of variation may 



usually be accumulated in successive generations by selection, 



without the other characters of the species being materially 



affected. 



may 



may 



which may have a habit of continually laying eggs ; and these 

 qualities are often obtained Avithout perceptibly changing in 

 any other respect the habits or organisation of the same 



races. 



In the case of the maize and the vine, we alter the seed 

 and the fruit without changing the leaves, whereas in the 

 foliage of the mulberry, cultivated for the sake of the silk- 

 worm, new varieties have been formed, the fruit remaining 

 the same. In the cabbage the leaves have undergone 

 wonderful transformations, as have the tubers in the potato 

 and the roots in the carrot, while the characters of the flowers 

 in all have remained unaltered. The modifications produced 

 in the seeds of the maize deserve especial notice. The 

 different races vary in height from eighteen inches to as 

 many feet, and the whole ear in one variety is more than 

 four times as long as in another dwarf kind. The seeds 

 are coloured white, pale yeUow, orange, red, violet, or 



