674 LIFE : OUTLINES OF "GENERAL BIOLOGY 



especially true of those that are closely bound up together in con- 

 nection with some function; thus if the breathing organs are not 

 doing their work sufficiently well, there will be a strain on the heart 

 to send more blood through them. There is also likely to be a corre- 

 lation between two structures that have developed and evolved 

 together, as in the case of the brain and the eyes, the stomach and 

 the liver. A new departure or variation due to some initial germinal 

 disturbance may bring a correlated variation in its train, especially 

 in a structure which is historically associated with the first. Or a 

 variation in the activity of a ductless gland may affect several dis- 

 tinct structures which respond to the particular hormone secreted. 

 The morphological aspect of the "correlation of parts" was promi- 

 nent in the work of Cuvier (1769-1832), to whom the idea was a 

 guiding principle, though he failed — indeed refused — to see its chief 

 import. What impressed him as a comparative anatomist was that 

 certain structures go together, or, on the other hand, that certain 

 structures exclude one another. If a mammal "chews the cud", it 

 will have a "cloven hoof"; if the embryo of a Vertebrate has an 

 allantois, it will not have gills ; if a fish is a cartilaginous fish, it will 

 have a spiral valve in its intestine ; if a reptile has a bony carapace, 

 it will not have a breastbone, and so on. Cuvier had a clear vision 

 of the fact that an organism is a unified integrate, but he exaggerated 

 terribly when he wrote: "A claw, a shoulder-blade, a condyle, a leg 

 or arm-bone, or any other bone separately considered, enables us to 

 discover the description of teeth to which they have belonged; so 

 also, reciprocally, we may determine the form of the other bones 

 from the teeth. Thus commencing the investigation by a careful 

 study of any one bone by itself, a person who is sufficiently master 

 of the laws of organic structure may, as it were, reconstruct the 

 whole animal to which that bone had belonged." What Cuvier 

 missed, being a determined opponent of Lamarck's heretical evolu- 

 tionism, was the illuminating idea that these correlations depend 

 on a common ancestry, in which similar variational changes have 

 taken place in different parts of the body. If we find a recent mam 

 malian skull with an inflected angle of the lower jaw, and with tht 

 jugal bone continued backwards to share in making the glenoid 

 fossa for the articulation of the lower jaw, we may safely make a 

 somewhat precise technical statement in regard to several pecu- 

 liarities of the teeth, even though they have all fallen out. For 

 Marsupials are the only animals which show in one skull the two 

 peculiarities first mentioned, and all living Marsupials that have 

 been studied have certain dental peculiarities. The correlation or 

 simultaneous occurrence of these two sets of peculiarities depends 

 on the presumed fact that Marsupials are descended from a common 

 ancestral stock, which probably had the beginnings at least of the 

 peculiar features. We do not doubt that there may be in certain 



