720 THE POPULAR SCIENCE MONTHLY. 



maiient change of structures and functions ; that a disturbing influ- 

 ence, even though it were to extend to many generations, can only 

 modify a race superficially ; and, finally, that, the instant that this 

 cause ceases to be, the race resumes, slowly but surely, its original 

 characters. 



In fact, the environment is ever changing, and in the enormous cycles 

 of changes in the conditions surrounding organic life upon the earth 

 the same conditions have never occurred a second time. Organisms 

 must follow this movement of variation ; they must be ever undergoing 

 a process of adaptation, in order to be in equilibrium with the altered 

 conditions around them. In this necessity for adaptation we recog- 

 nize a consequence of our first principles. The state of homogeneity 

 must give way to a state of heterogeneity : a species must be ever 

 growing more and more varied in its forms ; old species must be ever 

 breaking up into new. If at one time a species consisted of indi- 

 viduals alike in all respects, the action of the various forces of the 

 environment would soon put an end to this uniformity; at the same 

 time, however, leaving tokens of relationship. But let us go further, 

 and suppose the conditions to be still more profoundly altered, owing, 

 for instance, to a climatic perturbation of the habitat, or to an emi- 

 gration of the species into other habitats ; in that case there will be 

 difierent sets of conditions, and the groups of individuals will resem- 

 ble one another, or be unlike, according to the likeness or unlikeness 

 of the conditions. The connection between changes in the conditions, 

 changes in function, and changes in structure, is a consequence of the 

 persistence of force. 



The law of heredity, which is antagonistic to the law of variation, 

 may also be traced back to our first principles. This law represents 

 the element of fixity in the domain of life. All the organisms of a 

 given type are descended from organisms of the same type. If we 

 consider heredity in a succession of organisms, it appears to be inex- 

 plicable. Many still deny the existence of heredity, and explain the 

 resemblance of the child to its parentage by a special intervention of 

 the creative power of Nature. But, if we compare the heredity of 

 the individual with certain phenomena occurring in the individual, 

 for example, the repair of tissues, the reproduction of worn-out or 

 lost parts a process which in some animals goes so far as to repro- 

 duce liighly-complex organs or groups of organs (for instance, in liz- 

 ards, the reproduction of feet and tail ; the reconstruction of the 

 fresh-water hydra; the restoration of the plant Begonia 2:)liyllomani- 

 aca from a fragment of its leaf) we shall perceive that there exists a 

 tendency to reproduce like products, and that the two orders of phe- 

 nomena are related. We must suppose them both to be due to the 

 tendency of the physiological units of an organism to arrange them- 

 selves in the form proper to that organism. But we need not recog- 

 nize in this tendency any such mystic entity as an Archmus or a vital 



