HIGH SCHOOL ZOOLOGY. 289 



cells " of the Radiolaria, which seem to prosper in their hosts 

 and to occasion them no inconvenience by their presence. Pos- 

 sibly they furnish oxygen to the tissues, and utilize themselves 

 the waste products, including carbonic acid, excreted by the 

 animal tissues. 



28. The most obvious instiinces of the association of animals 

 and plants for mutual advantage are to be met with in the repro- 

 ductive phenomena of the vegetable kingd<im, chiefly in those 

 connected with the fertilisation and distribution of flowering 

 plants. It is now known that by far the greater number of 

 Phanerogams with conspicuous flowers are fertilized by ins ct- 

 agency, and tliMt the secretion of nectar, the colour and often 

 the form of the flower are so many inducements, acquired by 

 natural selection, to insects to visit them, while, on the other 

 hand, many peculiarities of form, or of the relative development 

 in time, of parts of the flower are so many obstacles to self- 

 fertilisation. So close is the i-elationship between flowering 

 plants and the insect-world that they may be said to have been 

 correlatively developed. 



Similar mutual advantages exist in the relation of frugivor- 

 ous birds to fruit-bearing plants; the distribution of such 

 plants has been shown to be largely effected by the birds in 

 question, and the bright cohnirs of fruits as well as their 

 sweetness receive a partial explanation in this way. Other 

 examples of the co-operatioti of animals in the distrilmtion of 

 plants are furnished by those whose fruits (burrs), being clothed 

 with hooks or sjnnes, adhere tenaciously to the coats of vai-ious 

 mammals, and thus secure a wider range. 



29. Such considerations as those above emphasize the interde- 

 pendence of the vegetable and animal kingdoms, and the neces- 

 sity of studying the phenomena of both in connection with each 

 other. This is rendered more impeiative when we come to 

 look at the economical aspects of Biology, and realize that the 



