THE PRESIDENT'S ADDRESS. 473 



From this point of view it is clearly impossible to establish 

 any definite criterion of individuality of general applicability, for 

 it is impossible to say exactly when a colony ceases to be a colony 

 and becomes an individual of a higher order. Our ideas of 

 individuality change completely as we review the animal kingdom 

 from the Protozoa upwards. 



It might be supposed that some light would be thrown upon 

 our problem by the study of the development of the individual 

 from the egg, and this is certainly a very profitable line of inquiry. 

 Can we say that we mean by an individual the whole undivided 

 body into which the egg-cell develops 1 We certainly can in many 

 cases, but there are many other cases in which we just as certainly 

 cannot. 



Let us return for a moment to the simple hydroid colony, as 

 we see it, for example, in Obelia or Sertularia. Here the fertilised 

 egg develops first into a single multicellular individual, but that 

 individual does not stop developing when it has attained its full 

 growth ; it branches out and produces other individuals by a pro- 

 cess of budding, and in the colony thus formed it is impossible to 

 say where one individual ends and another begins, though it may 

 be quite possible to tell how many individuals there are altogether 

 by simply counting heads. It is not, as a rule, until many non- 

 sexual individuals have been produced that some particular bud 

 develops into a new sexual individual which once more produces 

 eggs or sperm. Moreover, in this alternation of sexual and non- 

 sexual generations the two generations generally differ widely 

 from one another in structure, the sexual jelly-fish being strongly 

 contrasted with the non-sexual hydroid polype. 



A similar phenomenon of alternation or metagenesis occurs, of 

 course, in many other animals and in all the higher plants, 

 usually accompanied by great multiplication of the non-sexual 

 generation by some process of budding. An ordinary tree is the 

 non-sexual generation, and we can get as many individuals out 

 of it as we like by taking buds or cuttings, though we are 

 accustomed to look upon the whole tree as a single individual. 



A difficulty of quite a different kind is presented by the lichens, 

 which are well known to be composite organisms, made up of 

 combined algal and fungal constituents, and by the myxomycetes, 

 where the plasmodium is formed by the union of a number of 

 separate amoebulae. Here we get a number of individuals, 



Journ. Q. M. C, Series II. No. 76. 33 



