548 



SCIENCE. 



[N. S. Vol. XXII. No. 566. 



found on the East Indies. From the East 

 Indies we find lines diverging to the Red 

 Sea, to the Cape of Good Hope, to Japan, 

 to Polynesia and Hawaii, to Australia and 

 New Zealand. We, therefore, consider the 

 East Indies the center of dispersion or 

 primitive home for most types of tropical 

 fishes. At the Isthmus of Panama the 

 nearest relative of many species of fishes is 

 found across the isthmus, in the West 

 Indies. Thus the red snapper of Panama, 

 Lutianus Colorado, finds its analogue in the 

 red snapper of the Caribbean Sea — Luti- 

 anus aya. The large black snapper of 

 Panama, Lutianus novemfasciatus, finds its 

 nearest relative in the Cubera of the Carib- 

 bean, Lutianus cyanopterus. The same is 

 true of perhaps 300 other species at Pan- 

 ama, enough to show that the resemblance 

 across the isthmus is not a matter of acci- 

 dent. The divergence between these sepa- 

 rated forms dates apparently from the 

 time, in the late Miocene, when the seas 

 ceased to flow together across this isthmus. 



Again, in the Atlantic, the nearest rela- 

 tive of the Allice shad of the north of 

 Europe {Alosa finta) is the Thwaite shad 

 {Alosa rufa) of farther south. Another 

 near relative is the American shad {Alosa 

 sapidissima) of our Atlantic coast. This 

 again finds its nearest ally in the Gulf shad 

 (Alosa alabamm), and this once more in 

 the Ohio shad {Alosa ohiensis). 



Always the species nearest alike in struc- 

 ture are not found together, nor yet far 

 apart, and always a barrier lies between. 

 Where two closely allied forms are not 

 found to intergrade they are called distinct 

 species. If we find actual intergradation, 

 the occurrence of specimens intermediate 

 in structure, the term subspecies is used 

 for each of the recognizable groups, thus 

 connected. 



Thus of the bluebird, Sialia, we have in 

 America three main forms, the one boreal, 



found in northern and western mountain 

 regions, blue above and below alike, known 

 as Sialia arctica. Another form, blue 

 above and brown below, inhabits the whole 

 region east of the Rocky Mountains. This 

 is Sialia sialis, the common bluebird of the 

 east, ' with the sky on its back and the earth 

 on its breast. ' In the warmer parts of the 

 mountain region, Texas to California and 

 southward, is still another blue bird, much 

 like the second, but darker in color and 

 with a large patch of brown on the back as 

 well as on the breast. This is Sialia mexi- 

 cana. These three are considered as dis- 

 tinct species because there is no evidence 

 that intergradations occur in any part of 

 their range. One of these species, Sialia 

 mexicana, has been split up into subspecies, 

 and this is related to the fact that its range 

 is crossed by barriers greater than those 

 which check the movement of the others. 

 The typical Sialia mexicana is found in 

 Mexico. Sialia mexicana hairdi represents 

 it in the Rocky Mountain region, and Sialia 

 mexicana occidentalis, in the Pacific Coast 

 states. A local form, Sialia mexicana ana- 

 helce, is found on the mountain called San. 

 Pedro Martir, in Southern California. 

 Along the border line between Sialia sialis 

 and Sialia m,exicana, in Arizona and Mex- 

 ico, the eastern bluebird throws off shoots', 

 called Sialia sialis azurea. 



Widely distributed across the United 

 States and from southern Canada to Ari- 

 zona, we have the yellow warbler. Den- 

 droica cestiva. This bird is chiefly yellow, 

 olive on the back with chestnut streaks on 

 the sides, tail feathers colored like the body, 

 and without the white spot on the outer- 

 feathers shown in most of the other types, 

 of wood warblers composing the genus Den- 

 droica. 



The yellow warbler throughout its range> 

 is very uniform in size and color. Its 

 nearest relative differs in having a shade- 



