196 Causes and Course of Organic Evolution 



in their i)roper order (]). 44) the physiological activities of 

 irritabihty, nutrition, respiration, and growth. These four, 

 wlien in balanced and regulated activity, give health to the 

 organism, and ensure its ontogenetic growth. They also 

 represent the combined activity of heredity, environal action, 

 and proenvironal response or reaction. 



But amid plant and animal competitors, as well as amid 

 destructive physical agencies, the most successful or perfect 

 means of defense or resistance that the organism can evolve 

 \^'ill best insure its and its posterity's continued existence. 

 Every chemical substance therefore which may give strength 

 and resistance, particularly over an exposed surface; every 

 line of energy-flow that will conduce to transfer of defensive 

 material; every waste product of the cell-laboratory that would 

 in itself be useless as food, but which would, without being 

 deleterious to the organism, defend it against destructive 

 environal forces; every mode of growth that may be developed 

 to overcome denudation, laceration, dessication, burial, or 

 asphyxiation by inorganic agents; such will tend to be, and 

 are, perpetuated as helpful factors in the life of each organism. 

 Hence the origin, we believe, of the greatly varied cell con- 

 tents of plants (p. 158), as well as of plant and animal cuticles, 

 of crystals, and of not a few secretions amongst plants and 

 animals. Hence also the varied armature of many animals, 

 their coloration in part, the attitudes they often assume, and 

 the associations they may establish. 



So when successful vegetation and defense or resistance 

 coexist, so also will the best conditions be for perpetuation 

 of the life of the individual or of the group. A full recognition 

 of their joint importance in tlie history of each organism enables 

 us the better to appreciate the value of the factor which Wells, 

 Matthews, but specially Wallace and Darwin, emphasized, 

 and which we may shortly call "selection." This, like the 

 j3revious two factors, involves incessant energy-transfer or 

 dynamic change. So to speak, as some have done, of *'a theory 

 of dynamic evolution" is alike superfluous and tautological. 

 The very term evolution — a rolling out or unfolding — expresses 

 steady dynamical action. 



