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the one hand next to species that are in need of growth factors there 

 are closely related species which can do perfectly without, and that on 

 the other hand the need for a special growth factor is met with in very 

 divergent classes of plants and animals. However, it has been clearly 

 shown that the independence of the so-called auxo-autotrophic organ- 

 isms of the presence of growth factors in their food is not due to the 

 fact that they can do without these factors, but that they are able to 

 synthesize the compounds in question. This implies that the auxo- 

 heterotrophic cells have obviously lost these synthetic capacities, and 

 such a loss has apparently occurred at very different stages in the 

 course of evolution. 



Following Schopfer who has made so many important contributions 

 in this field, the situation may be sketched as follows. 



One gets the impression that at all levels of evolution one has to do 

 with a living substance that is essentially the same. But during the 

 trend of evolution this living substance has started to lose its independ- 

 ence, the losses in synthetic ability increasing with increasing mor- 

 phological complexity and increasing adaptation to a heterotrophic 

 life. Yet the main lines of metabolism have been maintained, the ulti- 

 mate requirements have remained unchanged. The differences be- 

 tween the various physiological groups of organisms are not to be 

 found in differences in the fundamental constitution of their living 

 substance, but just in the different ways in which the latter orig- 

 inates. Whether an organism itself synthesizes the active groups of its 

 enzyme systems, or has to find these groups ready-made in its medium, 

 is only of secondary importance : the final result being the same in 

 both cases. 



We may conclude from all this that the study of microbial growth 

 factors in close contact with that of vitamins have also led to an im- 

 pressive manifestation of a fundamental unity of metabolism in the 

 whole kingdom of life. 



On looking back on so many advances in our insight into the prin- 

 ciples underlying microbial life one may ask whether future research 

 will not necessarily be restricted to an extension and a consolidation 

 of these findings. It may appear that prospects for further develop- 

 ments of a more essential character are far from bright. 



In reality there is already ample evidence that future generations 



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