606 / \ilidily of the Doctrine of Alutation. 
BORRADAILLE, working with decapods, came to the con- 
clusion that great difficulty stands in the way of the ex- 
planation of specific differentiation by means of ordinary 
natural selection. 1 Mr. R. LAUTERBORN was so kind as 
to draw my attention to the appearance of the yellow 
Atropa Belladonna lutca, and to the evidence given on 
this variety by Dr. Scnuz. 2 Dr. RAATZ was also kind 
enough to send me some seeds of a most remarkable 
shiny brown variety of the sugar-beet which suddenly 
arose in the cultures of Klein- Wanzleben, and from which 
I obtained fine instances of the new character in my 
garden. 
I have only given a selection from the long list at 
my disposal. My object was simply to show that the 
doctrine of mutation already finds adherents everywhere 
and is supported by a broad foundation of facts. This 
justifies the expectation that the difficulties which still 
stand in its way will ultimately be overcome. 
7. THE EXPLANATION OF ADAPTATIONS. 
Ever since the belief in the common origin of organ- 
isms has been recognized as a basis for investigation and 
speculation, one aspect of the problem has aroused the 
special interest both of the author and of the student, viz., 
the explanation of adaptations. As a matter of fact this 
problem lies outside the scope of the present book, which 
is only concerned with the empirical foundations of the 
theory of descent. Nevertheless it seems to me that it 
1 L. A. BORRADAILLE, Marine Crustaceans, The Fauna and Ge- 
ography of the Maldive and Laccadive Archipelagoes, Vol. I. Pt. 2, 
page 197. 
2 Aintl. Bcricht iibcr die 33. Versamml. d. Natttrf. und Acrztc. 
Bonn, Sept., 1854 (Bonn, 1859), p. 139. A single specimen of the 
plant had been found a few years previously in the Black Forest. 
