256 Dynamic Theory. 



the animal is poisoned, unless very healthy and vigorous. They mul- 

 tiply like } r east in barley-wort. The blood from such dog, if injected 

 into another animal, will be found more poisonous than the first injec- 

 tion ; and each subsequent injection, and further remove from the first 

 inoculation is more virulent than the previous one. From this it is evident 

 the vibrios have undergone a change. Those from the last dog are different 

 from those from the first. They have become acclimated and newly adapted 

 to a new environment. In such cases as these the change of environment 

 from that which is ' 'appropriate" is not great, and the change in the organ- 

 ism is correspondingly small; but we must be prepared to expect that great 

 changes in the organism will follow important changes in the environment. 

 I have shown elsewhere how easily organisms of a high order are af- 

 fected while in their earliest embryonic condition, that is, their unicellu- 

 lar condition, or within a few removes from it. In fish hatching, when 

 the appropriate conditions are followed as closely as possible, the nor- 

 mal egg may produce such an abnormal fish as one possessed of two 

 tails, or two heads, or two backbones. Those organisms which unicellu- 

 lar to begin with never get much beyond that, are fully as liable, in 

 an inappropriate environment, to get a crooked start, and to reproduce 

 something different from themselves. Nor is this the assertion of a 

 new or exceptional doctrine. It is simpl}- the fundamental doctrine of 

 evolution that the organism is subject to its environment, changing 

 when it changes. I object to the term spontaneous especially spon- 

 taneous generation. There is no such thing. When an organism 

 "sports" and reproduces something different from itself, it is in conse- 

 quence of being under the influence of an environment different from 

 that which produced it. And conversely if the environment is differ- 

 ent the organism is bound to sport. The change may be great or little, 

 perceptible or imperceptible, but there is a change. But it follows fur- 

 ther, that if all the conditions are precisely the same, the organism will 

 " breed true. " And it was the ability to preserve uniform conditions 

 that enabled Pasteur, and many others after him, to cultivate and breed 

 different races of Bacteria. 



Of Contagious Diseases there appear to be two kinds, and they are 

 comparable in every respect to the two kinds of fermentation. The first answering to 

 the fermentation caused by active vital organisms, is likewise caused by vital organisms 

 which, starting either from the germ or from the mature organism, grow and reproduce 

 in the blood and tissues of the victim as parasites. The second sort acts like the soluble 

 ferments, and may be communicated by the living cell or by its remains or products. 



A consideration of some of these diseases will show that they come under the laws 

 governing organized bodies, and are subject to the same struggle for existence, and the 

 same development and modification by their variations of environment and habit. I 

 have, in chapter 25, given the history of several disease breeding parasites such as 

 Trichinae which are easy to be discovered. But those mentioned in this chapter were 

 not suspected to exist until comparatively lately, though of their effects sad experience 

 has long been had. 



