176 The Study of Animal Life PART in 



little more than two-layered cups of cells, the cavity of the 

 cup being the primitive food -canal. A parallel stage 

 occurs in the early life-history of most animals, when the 

 embryo has the form of a two-layered sac of cells, or is in 

 technical language a gastrula. Both in the racial and 

 individual life-history the formation of this primitive food- 

 canal occurs very early. But it is not certain that it the 

 primitive stomach was not at a still earlier stage an in- 

 ternal brood-cavity ! 



But instead of speculating about this, let us seek to 

 understand what is meant by the correlation of organs. 

 Certain parts of the body stand or fall together, they are 

 physiologically knit, they have been evolved in company. 

 Thus heart and lungs, muscles and nerves, are closely 

 correlated. Sometimes it is obvious why two or three 

 structures should be thus connected, for it is of the very 

 essence of an organism that its parts are members one of 

 another. In other cases the reason of the connection is 

 obscure. 



When organs either in the same or in different animals 

 have a similar origin, and are built up on the same funda- 

 mental plan, they are called homologous. Those whose 

 resemblance is merely that they have similar functions are 

 termed analogous. Even Aristotle recognised that some 

 structures apparently different were fundamentally the 

 same, and no small part of the progress of morphology has 

 consisted in the recognition of homologies. Thus it was a 

 great step when Goethe and others showed that the sepals, 

 petals, stamens, and carpels of a flower were really modified 

 leaves, or when Savigny discerned that the three pairs of 

 jaws beside an insect's mouth were really modified legs. 

 To Owen the precision of our conceptions in regard to 

 homologies is in great part due, though subsequent studies 

 in development have added welcome corroboration to many 

 of the comparisons which formerly were based solely on the 

 results of anatomy. Thus an organ derived from the outers? 

 embryonic layer cannot be homologous with one derived 

 from the innermost stratum of embryonic cells. Homo- : 

 logous organs in one animal are well illustrated by the 



