66 TEANSACTIONS AND PKOCEEDINGS OF THE [Sess. lxii. 



associated with the life of an exceedingly minute organism 

 affecting their roots, causing the development thereon of 

 nodules, in which they lived and brought about changes 

 that were favourable to the growth of their host. 



Instances in which two distinct organisms, each living its 

 own life, are associated together for their mutual advan- 

 tage, are not infrequent in the animal world, and the word 

 Si/moiosis has been coined by De Barry to express that 

 relationship. The classical researches of Darwin have 

 familiarised us with instances of symbiosis between animals 

 and vegetables, and have shown, with great wealth of illustra- 

 tion, how much plants are dependent on insects for their 

 cross fertilisation, and how whole species of plants may 

 be dependent for their existence upon the regular and 

 unfailing visitation of insects, and, on the other hand, how 

 certain species of insects are dependent on certain species 

 of plants for their food, which they obtain from no other 

 source. The visits of a determinate species of insect to a 

 certain species of plant may be without any benefit what- 

 ever to the plant. In that case, the relation of the insect 

 to the plant is simply that of a parasite feeding on food 

 provided by another, and giving nothing in exchange, and 

 in abstracting its food from its host, it may not only do 

 it some injury, but may even injure it so far as to kill it 

 outright. The various species of aphides are an example 

 of that kind. They are very numerous, and exhibit very 

 distinct characteristics according to the kind of plant on 

 which they feed. In how far species of aphis which are 

 found constantly associated with one genus of plants could 

 accommodate themselves to, and find means of subsistence 

 upon, other genera I do not know, but the probability is 

 that in few cases could the parasite subsist under the 

 altered conditions. In such a case the insect has some 

 modification of structure or of function suited to the 

 particular genus on which it lives: but what we require to 

 find in a case of symbiosis is, that the plant also has 

 acquired a modification of structure suited to the char- 

 acteristics of the insect which feeds on it. What we fail 

 to find in pure parasitism is mutual adaptation, with the 

 ultimate result of mutual advantage. Darwin and others 

 after him have shown how the forms of flowers in some 



