ZOOGENESIS 



only slightly variable; here it is always rather small 

 with about twenty arms which are always rather 

 short and stout. 



All of the most closely related feather-stars are only 

 slightly variable. But other feather-stars in widely 

 different groups both in the East and in the West 

 Indies show a comparable extraordinary variability. 



Extreme variability is characteristic of very many 

 creatures in the sea and in these variability is often 

 carried to what seem to be fantastic lengths. But 

 the creatures of the sea are known to relatively few 

 so that a detailed discussion of them would be un- 

 profitable. So we shall take our examples of vari- 

 ability chiefly from among the butterflies, as the vari- 

 ations of butterflies have been widely studied and are 

 therefore fairly well appreciated. Furthermore ex- 

 amples of variation in the butterflies may be seen in 

 any collection, and some, at least, are easily obtain- 

 able almost everywhere. 



One of our common butterflies varies from light 

 clear yellow with a wing spread of about xj inches 

 or rather less to deep brilliant orange with a bright 

 violet iridescence in the males and a wing spread of 

 nearly if inches. The males of the two extremes 

 have a different wing form and a wholly different odor. 

 There is no regular intergradation between the yellow 

 and the deep orange types. The butterflies are yel- 

 low, yellow faintly flushed with orange on the fore 

 wings, light orange, or deep orange. Intergrades be- 

 tween these color types are rare. The males of the 

 first two types are very fond of sitting on mud and 



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