236 Report S.A.A. Advancement of Science. 



given <uit again in the order in which they were received? A 

 moments reflection will shew that each successive stage in develop- 

 ment is the necessary antecedent of those which come after it. We 

 cannot, for example, have an eight-celled stage before a four-celled, 

 and so on. Every stage is strictly dependent upon and conditioned 



by that which immediately precedes it. Each stage, in fact, forms, 

 an important part of the complex stimulus which calls forth the next 

 stage, and the whole development is a process of progressive equili- 

 bration. The germ-cell will, therefore, commence its development 

 by dividing into two parts exactly as its ancestors did. and so long 

 as all the conditions remain the same the subsequent stages of the 

 ontogeny must all be identical with the ancestral stages, and must 

 vield an identical result. Thus, when a given sequence has once 

 been formed it must be repeated in the same way so long as the 

 environment remains constant, and the problem of ontogeny resolves 

 itself into the question : Why has organic evolution taken place at 

 all, and why have not all organisms remained in the condition of 

 simple primordial cells? or, looking at the question from a slightly- 

 different point of view, we may ask, what is the origin of those latent 

 forces in the germ-cell which determine its development into a parti- 

 cular species of plant or animal ? 



The answer to these questions appears to be that every time 

 the process of ontogeny is repeated the conditions under which it 

 takes place may be slightly different from what they were before,, 

 and the developing organism makes a definite response to the stimulus 

 of the changed environment. Structural modifications are thus pro- 

 duced in the individual by direct equilibration, or. in other words,, 

 new characters are acquired. 



It is obvious that, so long as the changed conditions are main- 

 tained these structural modifications must repeat themselves in each 

 succeeding generation, because the same causes will produce the 

 same effects, and in this way not only may new stages be added to 

 the end of the ontogeny but earlier stages may be modified, as we 

 see in the production of larval organs, etc. 



Suppose now that the changed environment returns to its 

 earlier condition, so that the stimuli which called forth the new 

 characters cease to operate. Will those new characters immediately 

 disappear or will they persist for a longer or shorter period? We 

 know they do not necessarily disappear in the lifetime of the in- 

 dividual, but will they be present in the next generation? A parti- 

 cular environmental stimulus may have been operating for, say, a 

 thousand generations, during each of which it has produced the same 

 effect in the ontogeny of some organism. Are we to suppose that 

 if that stimulus is suddenly removed the effect in question will at 

 once cease to show itself? This is what those who maintain thenon. 

 inheritance of acquired characters would have us believe. We pre- 

 fer to believe, and we have already given reasons for so doing, that 

 during the time for which the stimulus in question has been pro- 

 ducing structural modifications in the body by direct equilibration,. 



