OF PLANORBIS AT STEINHEIM. 15 



the retrogressive characteristics of the first and second sub-series could be compared with 

 the pathological conditions, normal and abnormal, of occasional diseased and senile 

 individuals of the progressive series, but that they were distinct as far as they showed that 

 whole series were affected. They were therefore spoken of as the results of normal 

 pathological conditions of the animal, which were inherited with ever increasing effect in 

 successive species, occasioning distortions and retrograde metamorphoses, and finally 

 leading to the extinction of the race. 



In the same words we can formulate the life history of a diseased individual, 

 since, as has been shown, a similar series of changes are produced in the forms and 

 characteristics of the diseased individuals of the progressive series, and that these, 

 though in a more confined field, are identical in their results, leading also to the death 

 of the individual. 



In the individual the effects are shown in the disturbance of the laws of growth 

 producing abnormal or premature weakness ; or in the natural exhaustion of the powers of 

 growth, causing senility. A wound and its results, whatever they may be, can unques- 

 tionalily be so classified, since it is primarily a severe shock to the system, which lays 

 additional burdens upon the powers of growth, and is usually followed, if severe, . by 

 retrogressive metamorphoses, or premature old age.^ Senility and its accompanying 

 metamorphoses also fall under the same law, though here there is no accident, and we 

 must refer it to the action of well known physiological laws. Thus, when the powers 

 of incremental growth during the life of any individual reach that point at which actual 

 increase in the size of its organs is no longer perceptible, physiology teaches us, that the 

 organs are maintained in size and the performance of their functions by an adequate 

 supply of nutriment ; but that, after a time, the individual becomes unable to digest 

 sufficient food to supply the waste occasioned by the performance of its functions. 

 Then, that those peculiar transformations take place, consisting of the loss of functions 

 and the gradual decrease in size and entire or partial absorption of parts and organs, 

 which constitute what are called the retrograde metamorphoses of old age. 



Senility, therefore, simply expresses the normal wearing out of the powers of vitalized 

 tissue to sustain itself against the perpetual friction with the disintegrating, wasting, and 

 idtimately unfavorable effect of existing physical surroundings. When we compare these 

 effects of unfavorable environment in producing distortions and decrease in size of the 

 individual, with the corresponding distortions and decrease in size of the retrogressive 

 sub-series, there is a certain similarity which leads to the supposition that the latter are 

 also probably due to an unfavorable environment. In other words, that the continuous 

 action of unfavorable environment upon a race, eventually produces variations in form 

 and characteristics in the successive but genetically connected species, which show that 

 their growth not only as individuals is interfered with, but that the distortions and 

 retrogressive characteristics thus produced tend to be inherited, and affect the whole 

 series of forms. 



We are justified, therefore, in assuming, that in all probability the sub-series were 

 retrogressive, because the environment in the Steinheim lake was so unfavorable ; that the 



1 The exceptions in which additional normal characteristics or abnormal ones are produced are very rare and 

 can be disregarded. 



