602 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



ganism and environment. The process of gene mutation itself, which pre- 

 sumably plays so important a role in evolution, is frequently the source of 

 maladjustments and therefore many mutations are lethal; in other instances 

 they may lead to malformations and abnormalities in metabolism and func- 

 tion, or they provide the basis on which certain environmental conditions can 

 act in an injurious way. Pathological mechanisms, or mechanisms arising in 

 response to injurious conditions and tending to counteract them, have their 

 phylogenetic history as well as normal mechanisms. We should therefore 

 be able to trace the origin and evolution of disease processes phylogenetically 

 in the same way as the evolution of normal structures and functions ; but only 

 the beginnings have been made in this direction. 



Thus, we have attempted to trace the development of thrombosis, a partial 

 or complete occlusion of blood vessels, which in mammals depends upon 

 changes in the vessels, in the character of the blood flow, and in the com- 

 position of the blood. From the primitive process of agglutination of amoebo- 

 cytes in Limulus and the subsequent combination of this with processes 

 of coagulation of blood in higher invertebrates, this condition ascends through 

 the lower vertebrates to its full complexity in mammals. But thrombosis, 

 which represents a disease and has destructive consequences for so many 

 higher organisms, is closely associated with processes which, instead of being 

 injurious, have an adaptive value, such as the prevention of bleeding follow- 

 ing injury. Furthermore, it is of interest in this connection that the mechanism 

 leading to the primitive thrombus formation as found in Limulus has much 

 in common with that underlying tissue formation, as we pointed out in a 

 preceding chapter. In a different field of pathology, Metchnikoff has shown 

 that it is possible to trace phylogenetically the activity of phagocytes, which 

 play so important a role in inflammation and in immunity, from simple proc- 

 esses of digestion in primitive organisms to the most complex reactions against 

 injurious material in mammals. 



In a provisional way we may distinguish four types of inadequacies or dis- 

 eases which are however not sharply separated from one another but overlap to 

 a certain extent. 1) The ultimate inadequacy, which becomes manifest in the 

 course of life in the differentiated and the rigid organisms belonging to the 

 higher classes of animals, is also the cause of the imperfect utilization of certain 

 important food factors, as is also the lack of tolerance by certain organs for 

 food factors which are necessary for other kinds of tissues or organs ; thus, 

 disharmonies in the organism set in. 2) Likewise, inadequacies in the relations 

 between the individual and his natural or social environment may lead to such 

 disharmonies and these, too, have had their evolution. 3) A third type of dis- 

 harmony causes a disease which may also be traced phylogenetically; it con- 

 sists in changes in certain tissues, which make them assume a cancerous growth 

 and thus invade and destroy the organism in which they originated. Cancerous 

 growth has, so far, been observed only in the relatively rigid organ and tissue 

 systems of the vertebrates; in those organisms, in which restitutive growth 

 processes lead to the formation of organs, to multiplication of the individual 

 animals, or to colony formation, excessive growth stimulation should not cause 



