504 INTRODUCTION TO EVOLUTION 



between large subdivisions of classification, such as orders and classes, are 

 seldom found. This situation is not true of reptiles and birds, where, as we 

 have seen, Archaeopteryx occupies an almost perfectly intermediate posi- 

 tion. Likewise, it is not true of reptiles and mammals, where the therapsid 

 reptiles grade into the primitive mammals almost insensibly. But it is true 

 in many instances. We have, for example, presented the insectivores as the 

 ancestral group from which the other orders of placental mammals arose. 

 There is good basis for doing so, yet no forms have been discovered that 

 are intermediate, for example, between insectivores and the flesh-eating 

 mammals of Order Carnivora, between insectivores and rodents (Order 

 Rodentia), or between insectivores and most other orders of mammals. 

 Does this lack mean that such intermediate forms never existed, that each 

 order arose by a sudden jump (saltation) from the insectivores? Or is the 

 lack simply due to the incompleteness of our knowledge of fossil animals? 

 Two schools of thought have arisen on this matter; the reader is referred to 

 Simpson (1953) for the pros and cons of the controversy. That author has 

 marshaled evidence in support of the view that intermediate or transitional 

 forms existed (recall that some have been found) but that most of them 

 remain unknown to us because of incompleteness of the geologic record. 

 He quoted ( 1949a) a pithy statement of the matter by H. E. Wood to the 

 effect that "the argument from absence of transitional types boils down to 

 the striking fact that such types are always lacking unless they have been 

 found." 



Why are such transitional types still largely absent from our collections 

 of fossils? The answer proposed by Simpson relates to the matter of rate of 

 evolution and constitutes the reason for bringing the subject of the gaps in 

 the fossil record into our discussion. The answer will summarize the best 

 conclusions which are available at the present writing concerning the causa- 

 tion of large evolutionary changes. To make the matter as concrete as pos- 

 sible, we shall present it in terms of a specific example, that of bats. Or- 

 der Chiroptera. Bats resemble insectivores in many ways but differ from 

 them by having wings. Bats have existed since at least the beginning of 

 the Cenozoic era, and the early bats had wings as well developed as are 

 those of their modern descendants. No transitional forms with partially 

 developed wings are known as fossils. If we grant that bats did not arise by 

 a single "systemic mutation" converting certain insectivores into bats 

 "overnight," how can we explain the observed facts? 



In the first place, it seems clear that bats did not arise by a long process of 

 accumulating slight changes in structure over a great span of time. Bat evo- 

 lution has been bradytelic in the extreme since early in the Cenozoic era, 



