PHYLOGENETIC RELATIONS OP ANIMAL GROUPS 879 



and the frog has the urostyle. Pythons and porpoises, neither of 

 which has the least use for them, have vestigial hind limbs similar 

 to the functional ones of their relatives. The salivary glands of 

 certain snakes have become adapted as poison glands, certain sweat 

 glands have become milk glands, and gill arches have become sup- 

 porting structures of the tongue, larynx, and throat of adult ter- 

 restrial animals. The blood supply and nerves both follow the 

 phylogenetic changes of these organs. 



Most animals seem not to have originated in these present forms, 

 and they do seem to have changed through the long periods of 

 geologic history. The explanation offered by modern biologists for 

 these anatomical relations and resemblances between animals is that 

 the individuals in any group have inherited a similar plan of struc- 

 ture from the ancestors which was common to all members of the 

 group. In a group, such as the vertebrates, there have been numer- 

 ous modifications of various fundamental structures in different 

 subdivisions in relation to the particular habits of life ; still they 

 remain fundamentally alike because they have developed from the 

 underlying plan of organization found in the ancestors. The seal 

 and the bird, although quite different, show similarities in habits 

 and otherwise because of common ancestry long ago. The conclu- 

 sion of biologists of today is that all of the animals in a group, such 

 as the vertebrates, have arisen by descent with change from a primi- 

 tive organism which possessed the fundamental organization as 

 shown from cyclostomes to man. 



Embryological Evidence. — Evidences from this field really con- 

 tinue directly from the previous discussion. The animal is to be 

 thought of as an individual from the single-celled zygote stage to 

 the mature stage of old age, no matter what its complexity. In- 

 timately related types of animals parallel through a large portion 

 of their development to diverge somewhat in adult condition, more 

 remotely related forms take separate developmental courses rather 

 early in life, and unrelated forms may be different almost from the 

 beginning. In numerous instances the developing stages of more 

 advanced forms resemble very closely the mature stages of the less 

 advanced types in a serial fashion. The history of the individual 

 animal often corresponds in a general way to the history of the 

 advances of the animal kingdom, up to its state of development. 

 This apparent repetition of the ancestral development in individuals 



