Evolution 329 



of the range cannot successfully breed. The fertilized eggs will begin 

 development, but will soon die. This is due to the fact that the normal 

 rate of development varies greatly among the individuals of different 

 parts of the range. Thus there is physiological incompatibility and 

 development is impossible. This difference is related to differences in 

 the environmental conditions : frogs from northern areas develop 

 more rapidly than do those from southern ones. This same incompati- 

 bility can be noted in the vertical distribution of R. pipiens in moun- 

 tainous areas. 



Possibly the various populations of R. pipiens represent incipient 

 species which have been separated long enough from one another to 

 develop important physiological differences, but not long enough to 

 become discrete species. 



Criticisms of Neo-Darwinism. — While Neo-Darwinism seems a 

 very logical explanation of evolution, one which even reconciles the 

 somewhat diverging evidence of genetics and Darwinian evolution, 

 it has been criticized by some geneticists. These scientists, Goldschmidt 

 in particular, have pointed out that no one has seen a clear-cut species 

 arise in this manner, and, further, the higher categories (phyla, etc.) 

 seem to be widely separated from one another. If Neo-Darwinism is 

 correct, there should be transitional forms between any two categories. 

 If these are not living today, they should at least be in the fossil record. 

 Moreover, Goldschmidt points out that any radical change in an animal's 

 mode of living such as flying, swimming, etc., requires that many changes 

 be made simultaneously. 



He suggests an alternative explanation : evolution at least of 

 the higher categories occurs by "jumps" which must involve whole 

 chromosomal rearrangeeients rather than single genes. Further, he 

 points out that in any population very aberrant forms are constantly 

 occurring. Most of these "monsters" die, but a few "hopeful ones" 

 survive to perpetuate themselves. This is in contrast to Neo-Darwinism 

 which believes that changes begin at the subspecies level and gradually 

 progress to the higher ones. Here the changes occur abruptly at the 

 higher levels and gradually appear at the lower ones. 



The Neo-Darwinists answer these objections by pointing out that it 

 takes a long time for species to arise and that genetics is a relatively 

 young science. There has not been suf^cient time to observe the pro- 

 duction of new species. Furthermore, domestic animal breeders have 

 developed forms which bear little or no resemblance to their ancestors. 



