474 Correspondence — C. Carus-Wilson. 



view. With the species of the sea-urchin ificrasiej- the changes undergone 

 in the course of ages were apparently gradual in all the different characters, 

 and many other instances of continuous evolution might be given. After 

 brief references to the concepts of seriation or the direction of change, 

 Dr. Bather proceeds to discuss the question of pre-determination, which 

 many pala3ontologists seem to have accepted. The i^rinciple is thus 

 expressed : "A race once started on a certain course will persist in that 

 course ; no matter how conditions may change, no matter how hurtful 

 to the individual its own changes inay be, progressive or retrogressive, 

 up hill and down hill, straight as a Roman road, it will go on to that 

 appointed end." The principle is allied to Driesch's concept of entelechy 

 in ontogenetic development, and the question of its validity seems to be 

 part of the wider problem as to whether teleological categories have any 

 place in biological thought, a question which is answered in the negative 

 by all men of science, excepting the small minority who adopt the attitude 

 of Dr. J. S. Haldane. Dr. Bather next discusses adaptive evolutional 

 " convergence " and passes on to consider Dollo's so-called law of 

 irreversible evolution. He does well to insist that this law is a statement 

 of observed fact, and, understood rightly, need not imply any inherent 

 principle affirming the impossibility of reversal. The next sections 

 deal with the " study of habitat " and" the tempo of evolution ", which, 

 it is shown, is variable, dej^ending on changes in the outer conditions. 

 This is illustrated by the comparative study of the development of whales, 

 sirenians, and horses during the Tertiary period, the rate of evolution of 

 these three groups varying with that of changes in the nature of the food. 

 Thus the sirenians underwent a slow change because they retained the 

 habit of feeding on soft water-plants, the horses developed more rapidly 

 in correlation with their taking to the plains and becoming eaters of grain, 

 while ^the whales, though at first their development was slow, from the 

 Oligocene onwards changed with extraordmary celerity as they adopted 

 and diversified new habits of feeding and living. In referring to the 

 " rhythm of life ", Dr. Bather concludes that the phenomenon must 

 reflect the great rhythmic waves that have uplifted the mountains and 

 lowered the deeps, as well as the smaller waves and ripples that have 

 from time to time diversified the face of the earth. The acldress is through- 

 out both suggestive and stimulating It concludes on a note of sanguine 

 anticij^ation for the future. 



F. H. A. M. 



CORRESPONDENCE. 



FLINT PROBLEMS. 



Sir, — All efforts to solve tlie problems of flint formation are likely 

 to be unsuccessful unless we marshal all known facts and deal with 

 them seriatim. At present I doubt if it is possible to give even a 

 satisfactory definition of flint ! 



Many years ago I found flint at Margate and Studland Bay, 

 which is clearly silicified chalk. In places silicification has not 

 been completed, and these softer parts have been removed, leaving 

 cavities. Near Corfe I came across nodules in the chalk which, 

 with the exception of a thin external crust of flint, are entirely 

 composed of soft, vfhite cryptocrystalline silica. There are flint 

 pebbles at Boscombe and Southbourne-on-Sea that are composed 

 partly of the same cryptocrystalline silica ^ and partly of chalcedonic 



1 Nature, May 1, 18 JO. 



