122 ANIMAL INDIVIDUALITY [CH. 



should ever give rise to a single individual of a higher 

 order than its members (see 5, p. 304). To them we 

 commend the example of the Siphonophora, and pass 

 on to consider some other individualities, formed 

 through aggregate differentiation, but after an en- 

 tirely new fashion. 



To start with, we have the old but ever interesting 

 fact of symbiosis, where two organisms as it were 

 inter-penetrate, entering into a very close relationship 

 from which both parties derive profit. The classical 

 examples of symbiosis are the Lichens, which, long 

 supposed to constitute a distinct group of plants, 

 were in the middle of last century discovered to be 

 actually a mixture of two organisms, one a colour- 

 less fungus, and the other a green plant a simple 

 alga. For the details of their organization any text- 

 book of botany can be consulted : here it must 

 suffice to say that there is a perfectly definite arrange- 

 ment of the algal and fungal constituents. The in- 

 teresting thing about them is that they will grow, as 

 anyone can see for himself, in situations which no 

 other plant would tolerate, so that both plants must 

 obviously derive advantage from the combination. 

 Put very briefly, the facts are these : fungi can only 

 get the carbon of their food from organic matter, 

 while green plants have the power of using the 

 energy of light to appropriate carbon from the 

 carbon dioxide of the atmosphere. In respect of 



