THE DEVELOPMENT OF ORGANISMS 807 



detected by the ultra-microscope and being seen by the microscope 

 which may be understood by pondering over the way a beam of 

 light shows us the usually invisible motes in a shaded room. 



How do microbes cause death? This is one of those readily asked 

 questions which require a volume for their answer, but we suppose 

 that an expert in these matters would broadly say that they often 

 make holes in tissues, and that they often poison the body or some 

 part of it with their waste-products. In other words, they act as 

 larger parasites may do, such as some threadworms which not only 

 cause lesions, but exude toxins. Speaking of threadworms reminds 

 us of the remarkable work of Prof. Fibiger, who recently received a 

 Nobel Prize. He showed that the presence and pressure of a thread- 

 worm, called Gongylonema, in the rat's stomach induces a form of 

 cancer; and the same sort of consequence occurs, at the lagoon 

 called the Kurisches Haff, in people who eat imperfectly cooked fish 

 containing a certain parasite. 



While some parasites pursue their characteristic drifting life of 

 ease, like somewhat passive unpaying boarders, with an impoverish- 

 ing and an evil influence, there are others which make aggressive 

 attacks on their host, sucking the blood like the hookworm, or 

 actively destroying red blood corpuscles like the malarial organism, 

 or moving about in the body like the larvae of warble-flies, and so 

 on. There should be some way of verbally distinguishing these 

 aggressive forms from the typical parasites. They are, as a matter 

 of fact, beasts of prey that work from the inside instead of from the 

 outside. 



The third kind of death is natural death — the result of the slow 

 mounting up of fatigue-effects, especially in hard- worked organs 

 like liver and kidneys, heart and brain. Natural death is due to the 

 accumulation of the unrecuperated results of wear and tear. Recent 

 investigation, especially that of Prof. Child, seems to show that it 

 is not the living matter that gets tired, but the furniture of the 

 laboratory, so to speak. The less labile framework of the cells, within 

 which the protoplasm works, becomes gradually worn. No doubt 

 there are processes of repair and recuperation — in nutrition, in rest, 

 in sleep, in change, and, in some lower animals, in remarkable 

 processes of tissue-scrapping followed by reconstruction. 



But in the long run senescence gets the better of rejuvenescence, 

 and the organism "dies a natural death". In many cases a slight 

 environmental gust, like an unusually cold wind, sweeps the aged 

 player off the stage. But though the pimctuating full stop may be 

 inserted violently, the real cause of death is "natural". "And so, 

 from hour to hour, we ripe and ripe; and then, from hour to hour, 

 we rot and rot. And thereby hangs a tale". 



The only animals that escape natural death are some of the 

 Protozoa. They are so simple in structure that they seem continually 



