130 BIOLOGY AND ZOOLOGY 



the bacteria; and others again as an entirely separate group of unicellular organisms 

 showing little affinity either with bacteria or with any group of Protozoa. The balance 

 of evidence certainly points to their exclusion from the animal kingdom. 



Protozoa (General). -Dobell has raised an interesting but fine-drawn point in classi- 

 fication, relating not only to Protozoa but to all so-called unicellular organisms, animal, 

 vegetable and on the borderland. The definition of a cell, he says in effect, is that it 

 is part of an organism. Metazoa and Metaphyta are certainly cellular organisms, 

 inasmuch as their bodies are built up of cells. But Protozoa and Protophyta are not 

 unicellular, inasmuch as they are not cellular organisms at all. So also although 

 gametes are cells, specialised cells of multicellular organisms, the fertilised egg-cell 

 is not a cell but a whole organism. 



E. A. Minchin, in a valuable new treatise on the whole group of Protozoa (see also 

 E. B. xxii, 479 et seq.}, discusses a similar problem from a wider standpoint. He 

 suggests that there are three stages in the evolution of organisms. First there is the 

 evolution of the cell itself from some primitive organism, such as for instance the 

 Chlamydozoa, if these prove to be'real, independent entities. In such evolution the 

 cytoplasm and chromatin became distinct, and the chromatin became differentiated 

 into a nucleus. The second s^age was a further elaboration of the cell, with perfection 

 of the processes of nuclear division and the establishment of syngamy and true sex. 

 Only after these would have come the third stage, the evolution of ordinary multi- 

 cellular animals and plants in different directions. In the same treatise, following 

 separately published memoirs by himself and others, Minchin discusses the relations 

 between parasitic and free-living protozoa. 



Much of the brilliant work on pathogenic Protozoa has been carried out by medical 

 men and specialists, acquainted chiefly with the forms that cause disease. From 

 the broad point of view, parasites are abnormal and aberrant members of their race, 

 and their life-history cannot be understood without knowledge of their normal rela- 

 tives. Many parasites are new products of evolution. There is reason to believe 

 that Trypanosoma rhodesiense, a trypanosome fatal to human beings, has come 

 into existence quite recently, and many others show the marked disharmony with 

 their surroundings that is characteristic of unsettled form and changing habit. From 

 the vast assemblage of Protozoa that live free lives in the mud or water or air, some, 

 frorn time to time, find their way into the bodies of living creatures and contrive to 

 maintain existence in their new quarters. It can seldom happen that such intruders 

 are by accident so nicely adapted to a particular host that they can live in it and in 

 no other. They show a general power of tolerating parasitic life by being able to 

 live in many different kinds of animals. But they also show a want for adaptation to 

 their new habitat by multiplying unreasonably, and causing fatal disturbance in the 

 body of their host. Plainly it can be no advantage to a parasite to kill its host, so 

 destroying the living substance on which it subsists, and thus in course of time, partly 

 by modification of the parasite and partly by modification of the host, a mutual tolera- 

 tion is acquired. The protozoon becomes accurately adapted to life in a particular 

 host and causes it little or no harm. Parasites of such long-settled habit abound 

 in all kinds of terrestrial and aquatic animals, but occasionally it happens, by a new 

 intrusion from the stock of non-parasitic creatures, or by the migration of a possible 

 host into a new country, that cases of organic disharmony occur, with a consequent 

 catastrophic sweep of disease. 



Isle of Wight Disease of Bees. The cause of this disease, which has been extremely 

 destructive in recent years, has been discovered by H. B. Fantham and A. Porter. It 

 is a minute microsporidian parasite, named by its discoverers Noscma apis, and infest- 

 ing the alimentary tract of the bee. A spore swallowed by a bee becomes amoeboid 

 and enters one of the epithelial cells lining the gut. It then becomes nearly spherical, 

 feeds, grows actively, and multiplies by binary fission in several ways, producing clus- 

 ters or chains. The presence of the parasite deranges the digestive activities of its 

 host, and is frequently fatal. Ultimately sporogony takes place and each of the pan- 



