ELEMENTARY VITAL PHENOMENA 179 



vertebrates, are the result of manifold and profound transforma- 

 tions, while others, such as the Protista, have persisted from the 

 earliest times in a form changed relatively little. The last fact, 

 that in the unicellular Protista there is recognised a group of 

 organisms that possess in almost absolute purity the characters of 

 the ancient ancestors of all organisms, makes these micro-organisms 

 appear particularly valuable physiologically. But let us go some- 

 what more fully into the phenomena of the development of form in 

 general. 



No substance exists without form. All substance has a definite 

 form which is the expression of chemico-physical laws that pertain 

 partly to the nature of the substance in question and partly to the 

 influences that it receives from the outside. Living substance is 

 only a portion of the matter that composes the earth, and is not 

 different in its elementary nature from other substances. In 

 assuming form, therefore, living substance must obey the mecha- 

 nical laws of matter, as all other bodies do. If an organism has a 

 definite form, however, there are two factors, the mutual working 

 of which determines its further form-development a conservative 

 factor, which acts to maintain the form, and a mutative factor, 

 which acts to change it. The factor that maintains form is the 

 inheritance of present characteristics, the factor that causes change 

 is adaptation to changed external conditions. 



1. Heredity 



Heredity is one of the most familiar phenomena, so familiar that 

 in daily life we scarcely notice it and become conscious of it only 

 in special cases. By heredity is meant simply the fact that in 

 reproduction characteristics of the parents are transmitted to the 

 offspring, so that the descendants resemble in general the 

 ancestors. The offspring of a beetle become beetles of the same 

 form, and from the eggs of a fowl fowls develop ; a dog can produce 

 only a dog, a human being only a human being and never other 

 species. This transmission of the characteristics of the parents to 

 the offspring pertains to the minutest details ; not only is the 

 external form of the body transmitted, but special peculiarities of 

 motion, attitudes, habits, etc. This is seen most clearly in human 

 beings, since by practice in distinguishing them our gaze is 

 sharpened even for minutiae. But, as a rule, the fact of heredity 

 strikes us only when it has to do with specially characteristic 

 signs, when we see transmitted from parents to children peculiar 

 features, abnormalities of the body, such as supernumerary 

 fingers, hair over the whole body or upon unusual parts, and physical 

 defects. 



But not all peculiarities are always inherited. Many special 

 characteristics are not inherited at all, others are transmitted 



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