GREAT STEPS IN ORGANIC EVOLUTION 861 



heterogeneous group of Mesozoa, e.g. Dicyema, Rhopalura, and 

 Salinella, which occupy a midway position between Protozoa and 

 Metazoa. Some are of great simplicity; thus Salinella consists of a 

 single-layered tube of ciliated cells, open at both ends; but the 

 majority are parasites, and it is therefore possible that their simplicity 

 is in part retrogressive, not primitive. 



NATURAL DEATH. — On the acquisition of a body a penalty was 

 imposed — namely. Natural Death. In other words, the Protozoa 

 seem to enjoy cellular immortality. From time to time, nowadays, 

 they exhibit symptoms of wear and tear and loss of vigour, periods 

 of physiological "depression", as it is called; but in natural conditions 

 they rally and continue as before insurgently — moving, feeding, 

 growing, and multiplying. Hence Weismann's striking phrase — • 

 "the immortality of the Protozoa". 



Here a distinction must, of course, be drawn between the three 

 chief kinds of death, {a) If death be the irrecoverable cessation of 

 protoplasmic activity, one of its commonest forms is violent, as when 

 the organism or some important part of it is shattered or devoured, 

 frozen, or dried up. Most living creatures die a violent death, and 

 from this there is no exemption for the Protozoa, or for any other 

 unicellular form of life. They may be crushed, smothered, devoured, 

 and so forth, though their minuteness is perhaps of some survival- 

 value. It is probable that vicissitudes of temperature account for 

 the death of the coimtless myriads of unicellulars which sink slowly 

 from the surface to the floor of the sea, where they form the funda- 

 mental food-supply of abyssal animals. In any case, Protozoa are 

 certainly not exempt from violent death. 



The second kind of death is microbic and macro-parasitic, when 

 the organism is disintegrated or punctured or poisoned by virulent 

 intruders. These may be Bacteria, as in plague and tuberculosis; 

 or aggressive Protozoa, as in Malaria and Sleeping Sickness; or 

 multicellular intruders, such as Hookworm and Bilharzia. The 

 damage done by the invaders is varied ; it may be a breaking down 

 of tissue, or a choking of narrow passages, or a making of punctures; 

 but the commonest injury done by the small forms (microbes) is of 

 the nature of poisoning. Thus the malaria parasite (Plasmodium) 

 destroys red blood corpuscles; but in so doing it liberates toxins 

 into the circulation. Death due to microbes or to larger intruders is 

 very common in man and in his stock, partly because of the artifici- 

 ality of the conditions of life, and partly because of the frequent 

 lowering of powers of resistance, e.g. the vigour of the bodyguard 

 of amoeboid cells or phagocytes, which are normally useful in 

 engulfing and digesting intruding Bacteria. In Wild Nature microbic 

 disease is very uncommon, and even such cases as a bacterial disease 

 in sandhoppers, and another in salmon, may be associated with 



