Development | 67 



this sequence could be modified without throwing it lethally out of 

 balance. For example, gill pouches (embryonic precursors of gill 

 slits in fishes) became altered into other structures, such as eusta- 

 chian tubes and thymus glands, in higher vertebrates. This course 

 of evolution avoided the possible complications which might have 

 resulted from altering the entire set of processes producing the 

 pouches themselves. Such alteration might well have caused great 

 disturbance in the inductive systems responsible for, say, the devel- 

 opment of the aortic arches. 



The idea of recapitulation involves resemblance of developmental 

 stages to ancestral forms. However, there are some cases in which 

 adult forms appear to be similar to embryonic stages of their puta- 

 tive ancestors. For example, the females of some moths and beetles 

 are larviform; certain salamanders do not metamorphose into adults 

 but reproduce as larvae. Many characteristics of adult human beings 

 (relative hairlessness, large head, etc.) are reminiscent of those of 

 young anthropoid apes. The milk teeth of Australopithecus, the 

 earliest known fossil man, resemble the adult teeth of Homo sapiens, 

 while the permanent teeth of Australopithecus are like those of apes. 

 In these and in a great many other similar cases, evolution seems to 

 have altered the developmental system so that an intermediate 

 ancestral growth stage becomes the terminal form in the descendant. 

 This phenomenon is known as neoteny. 



The sequence of stages in the development of an individual organ- 

 ism often is thought of as merely steps toward a final goal: the 

 adult. It is surely more realistic biologically to think of ontogeny as 

 the continuallv changing response of a given body of genetic mate- 

 rial to a given environment. The various processes of the epigeno- 

 type regulate in varying degree the expression of the initiating geno- 

 type. Evolutionary change may involve any of the arbitrarily de- 

 limited stages of development. 



SUMMARY 



A line of descent does not consist of a straight-line sequence of 

 individuals but of a series of cyclic phases. Each complete cycle is a 

 developmental sequence, traditionally thought of as extending from 

 the beginning of one diplophase (zygote) to the beginning of the 

 next. Changes in the genetic information cause a variation in de- 

 velopmental sequence, and the accumulation of these genetically 

 initiated changes constitutes evolution. It is important to remember 

 that the entire life cycle evolves and that all stages of any given 

 cycle are essential to survival and thus equally important from the 



