No. 610] GENETICS VERSUS PALEONTOLOGY 



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greater or lesser degree by environmental conditions. It 

 is certain that changes in the conditions of life are not 

 the sole causes of modification, it is highly probable that 

 the chromosomes are insensitive to most somatic reac- 

 tions to the environment ; yet how can the student of the 

 Cetacea, who sees how thoroughly the ancestral quadru- 

 pedal heritage has been overlaid by the fish-like habitus, 

 doubt that in the end, and perhaps in some very indirect 

 way, the pelagic environment has conditioned the line of 

 evolution of the cetacean chromosomes, as it plainly has 

 conditioned the evolution of cetacean cytoplasm. And 

 when similar adaptations are produced among widely 

 separate stocks, it can scarcely be doubted that the similar 

 results are due to the similarity of the external conditions 

 as well as to the fundamental similarities of all cyto- 

 plasm and of all chromatin. Hence, without any com- 

 mitment as to the mode of evolution, paleontologists adopt 

 the principle of progressive and retrogressive adaptation 

 to environmental conditions as sufficiently demonstrated. 

 And most paleontologists would probably recognize that 

 the foot, for example, is just as much a part of the en- 

 vironment of the femur as is the medium upon which the 

 foot rests, in other words that evolution of a given struc- 

 ture is conditioned by its internal environment as much as 

 by external environment. 



Yet such is the skepticism which sometimes results from 

 modern studies in genetics that I have known graduate 

 students who seriously doubted the reality and value of 

 the principle of progressive and retrogressive adapta- 

 tion, on the ground that, as natural selection and the in- 

 heritance of ''acquired" characters had both been dis- 

 proved, there was no conceivable moans whereby adapta- 

 tion could be brought about ! But if tlio>o sko{>ti('s would 

 study for example the evolution of Triassic ganoids into 

 Cretaceous and modern teleosts, if they would consider in 

 detail the structural improvements in the locomotive ap- 

 paratus of teleosts, which involve the transformation of 

 scales into dermal rays, or of a heterocercal tail into a 

 homocercal tail, or if they would examine the evidence 



