94 



THE AMERICAN NATURALIST [Vol. XLIII 



strong an attack on the theory as does Montgomery. In 

 his "Unsere Korperform " as translated and quoted by 

 Morgan ('03, p. 71) who does not, however, assent. His 

 says : 



In the entire series of forms which a developing organism runs 

 through, each form is the necessary antecedent step of the following. 

 If the embryo is to reach the complicated end forms, it must pass, 

 step by step, through the simpler ones. Each step of the series is 

 the physiological consequence of the preceding stage and the necessary 

 antecedent for the following. Jumps, or short cuts, of the develop- 

 mental process, are unknown in the physiological process of develop- 

 ment. If embryonic forms. are the inevitable precedents of the mature 

 forms, because the more complicated forms must pass through the 

 simpler ones, we can understand the fact that paleontological forms 

 are embryonal, because they have remained at the lower stage of 

 development, and the present embryos must pass also through lower 

 stages in order to reach the higher. But it is by no means necessary 

 for the later, higher forms to pass through embryonal forms because 

 their ancestors have once existed in this condition. To take a special 

 case, suppose in the course of generations a species has increased its 

 length of life gradually from one, two, three years to eighty years. 

 The last animal would have had ancestors that lived for one year, two 

 years, three years, etc., up to eighty years. But who would claim that 

 because the final eighty years species must pass necessarily through 



year, two years, three years, etc.? The descent theory is correct in 

 so far as it maintains that older, simpler forms have been the fore- 

 fathers of later, complicated forms. In this case the resemblance of 

 the older, simpler forms to the embryos of later forms is explained 

 without assuming any law of inheritance whatever. The same re- 

 semblance between the older and simpler adult forms would remain 

 intelligible were there no relation at all between them. 



There are two ways of looking at this view of His's that 

 every form is the necessary antecedent of the succeeding. 

 These depend upon the length of stages considered. If 

 we take stages separated by very small intervals of 

 growth, His's contention must be true else there would be 

 no continuity of development. But this is nothing more 

 than a statement of the fact that all growth must be grad- 

 ual and is no law of development. If instead of small in- 

 tervals we take the whole development, the statement 



