64 EVOLUTION AND ANIMAL LIFE 



Huxiey carries this calculation still further by showing that 

 the number of cats depends on the number of unmarried women. 

 On the other hand, clover produces beef, and beef strength. 

 Thus in a degree the prowess of England is related to the number 

 of spinsters in its rural districts! This statement would be true 

 in all seriousness were it not that so many other elements come 

 into the calculation. But whether true or not, it illustrates the 

 way in which causes and effects in biology become intertangled. 



There was introduced into California from Australia, on 

 young lemon trees, twenty-five years ago, an insect pest called 

 the cottony cushion scale (Icerya purchasi). This pest in- 

 creased in numbers with extraordinary rapidity, and in ten years 

 threatened to destroy completely the great orange orchards of 

 California. Artificial remedies were of little avail. Finally, an 

 entomologist was sent to Australia to find out if this scale insect 

 had not some special natural enemy in its native country. It was 

 found that in Australia a certain species of ladybird beetle 

 attacked and fed on the cottony cushion scales and kept them 

 in check (Fig. 39). Some of these ladybirds (Vedalia cardi- 

 nalis) were brought to California and released in a scale-infested 

 orchard. The ladybirds, having plenty of food, thrived and 

 produced many young. Soon they were in such numbers that 

 many of them could be distributed to other orchards. In 

 two or three years the Vedalias had become so numerous 

 and widely distributed that the cottony cushion scales began 

 to diminish perceptibly, and soon the pest was nearly 

 wiped out. But with the disappearance of the scales came 

 also a disappearance of the ladybirds, and it was then dis- 

 covered that the Vedalias fed only on cottony cushion scales 

 and could not live where the scales were not. So now, in order 

 to have a stock of Vedalias on hand in California, it is necessary 

 to keep protected some colonies of the cottony cushion scale 

 to serve as food. Of course, with the disappearance of the pre- 

 daceous ladybirds the scale began to increase again in various 

 parts of the State, but with the sending of Vedalias to these 

 localities the scale was again crushed. How close is the inter- 

 dependence of these two species! 



There is little foundation for the current belief that each 

 species of animal has originated in the area it now occupies, for 

 in many cases our knowledge of palaeontology shows the reverse 

 of this to be true. Even more incorrect is the belief that each 



