ANIMAL EVOLUTION 



5*7 



as typical wc consider it aberrant or 

 abnormal. 



But in the animal world no forms arc 

 really aberrant or abnormal. The indi- 

 viduals we regard as such are merely 

 unusual under present day conditions. 

 Let conditions change, and a so-called 

 abnormal form in any animal may prove to 

 be better fitted to exist and so may replace 

 the present normal form. This may 

 happen in the adults only, or in any or 

 all of the younger stages. 



As the chemical and physical relation- 

 ships of animals are of two kinds, internal 

 and external, variation may be induced 

 either by internal or external causes, or by 

 both together. 



The significance of variation may be 

 made clear by the citation of a few 

 examples. 



Among the vertebrates there are two 

 well marked lines of deviation from the 

 structural average, both of which are 

 characteristic of certain types and also 

 occur as deformities or abnormalities in 

 others, in which they are incompatible 

 with other features necessary for existence. 



In all the back-boned animals there is a 

 marked tendency toward a great enlarge- 

 ment of the hinder pair of limbs, with a 

 corresponding reduction in the size of the 

 anterior pair. This feature is characteris- 

 tic of the frogs, dinosaurs, moas, ostriches 

 and other birds, certain fishes, and such 

 mammals as the kangaroos, rabbits and 

 hares, jerboas and jumping mice, and 

 others. It is a not infrequent "deform- 

 ity" in cats and cattle, and probably in 

 all other vertebrates; but in these it only 

 appears occasionally, and the individuals 

 so malformed usually die young, being 

 unable to meet the competition of their 

 relatives of the usual type. 



A study of the malformed young of cats 

 and cattle shows that in these animals 

 there is a latent tendency to develop types 



resembling the rabbits or the kangaroos. 

 A kangaroo-like cat, however, could not 

 hold its prey, while the feet of a kangaroo- 

 cow would be wholly unsuitcd for a 

 leaping mode of progress. Nature con- 

 stantly is striving to produce these crea- 

 tures, which in the modern world arc 

 impossible anomalies. 



Enlargement of the fore limbs at the 

 expense of the hinder pair is characteristic 

 of such birds as goatsuckers, frigate-birds 

 and others, of the pterodactyls, of flying 

 and certain other types of fishes, and of 

 bats, most monkeys, and certain other 

 types of mammals. 



Among the butterflies individuals occa- 

 sionally are found which are male on one 

 side and female on the other. Sometimes 

 the wings of a single individual arc 

 divided into irregular or more or less 

 regular patches some of which show the 

 male and others the female type of colora- 

 tion. Many cases of hermaphroditism 

 are also known from other insects, and 

 from all the groups of vertebrates. 



Hermaphroditic individuals in bisexual 

 forms are abnormal only when judged by 

 the standard of their parents. Viewed in 

 their broader aspect they are not abnormal, 

 but represent a recrudescence of a tendency 

 everywhere present among the animals to 

 unite both sexes in a single individual. 



Such bisexualism is a characteristic 

 feature of the individuals in a number of 

 different types of animals, where its 

 fixation as a normal feature is not incom- 

 patible with other economically necessary 

 features. 



Extreme variants range all the way 

 from frequent to very rare. But every 

 type of variant capable of existence seems 

 to be recurrent in succeeding generations, 

 and it has been found that some, at least, 

 are strongly dominant when bred with the 

 usual form. 



The occurrence of many different kinds 



