40 Mac LEOD, The Place of Science in History. 



and develop little by little, and at the end of a sufficient 

 time they are perfect enough to allow the animal to make 

 out the objects which surround it. 



The common Snapdragon {Antirrhinum majus) bears 

 strange flowers which resemble a mask or a muzzle. But 

 there also exists a variety of snapdragon which has (like 

 many other plants) a star-shaped flower of five petals. If 

 the two varieties are crossed it is possible to obtain hybrids 

 which bear flowers of the mask form when well nourished 

 and star- shaped flowers when ill nourished. 



The beautiful colours of butterflies, which are charac- 

 teristic of each species, can be profoundly modified by 

 transferring the caterpillars to plants other than those to 

 which they are accustomed. The colours of butterflies 

 can also be modified by exposing the chrysalis to a tem- 

 perature differing from the ordinary temperature. 



The tropical plant called Philodendron pertusum bears 

 large leaves pierced with round holes, which give to this 

 curious plant the name of pertusum, which means per- 

 forated. When this plant is cultivated in a greenhouse 

 sufficiently cold and dry, the holes do not form : the per- 

 tusum ceases to he perforated. In a sufficiently warm and 

 wet greenhouse, however, the holes reappear. 



These few examples, chosen from amongst a large 

 number, suffice to show that living things are, at least in a 

 large measure, shaped, formed by external causes. THIS 

 CALLS FOR REFLECTION. We say that the copper-beach 

 has red leaves and that the olm is blind because we have 

 seen these things from generation to generation in the 

 same conditions, but they are not necessarily so (in other 

 words, it is not a la7u } but only a rule that they are so). 



Man does not escape the common rule. In his bodily 

 as in his spiritual attributes he bears the mark of the ex- 

 ternal influences which he has suffered. The action of 



