114 
morphology, i.e. their genetic relationships as ex- 
pressed by their structure. In working out these 
relationships, as every morphologist knows, it is 
essential to have due regard to structure as a whole, 
collecting and weighing the evidence afforded by 
all the various organ systems of the body. The 
group name Dipnoi, or Amphibia, or Reptilia, or 
Aves, or Mammalia, connotes in each case a par- 
ticular assemblage of structural characteristics relating 
to the entire structure of the body. 
Now it is particularly desirable to bear in mind 
that when an extinct animal is allocated to one of 
the larger classificatory groups, this is done as a 
rule on no more sure basis than a knowledge— 
often a very imperfect knowledge—of the inorganic 
portions of its skeletal system, and consequently 
such allocation is, as regards the probability of its 
being correct, on a totally different footing from 
the assignment of a modern animal to its taxonomic 
group after full consideration of its whole structure. 
It is quite impossible for any one to say whether a 
palzeozoic creature now included in the group Dipnoi 
or Crossopterygii would, or would not, have this 
inclusion justified were we acquainted with its 
general structure apart from the skeleton. The 
same consideration indicates to us how vain were 
the old controversies as to whether the ancestor of 
the group Mammalia was an amphibian or a reptile. 
Even had we before us the undoubted skeleton of 
that ancestor in perfect condition, we should still 
require to know about its soft parts—its skin, its 
heart, its main blood vessels, its brain, its urino- 
genital organs, its embryonic membranes, and so 
on—before we should be justified in concluding 
definitely in which, if either, of the two groups 
named it should really be included. 
J. GRAHAM KERR. 
The University, Glasgow, December 19. 


Some Interesting Tracks of Alpha Particles in 
Gases. 
SELECTED photographs taken from about ten 
thousand exposures show a number of types of alpha 
ray tracks, some of which have been described before 
and some have not. Fig. 1 gives a track in which 

it is apparent that the alpha particle hits the nucleus 
of an oxygen or nitrogen atom. The nucleus is 
projected forward at a very high speed, while the 
alpha particle is reflected backward at a sharp angle. 
In Fig. 2 the track is an almost straight line with 
a branch which goes off at an angle of about 8°. 
In some instances the branch is at an angle as 
great as 50° with the straight track. An example | 
NO. 2778, VOL. 111 | 
NATURE 
[JANUARY 27, 1923 
of this is given in Fig. 3, though in the plane of the 
photograph the angle is only 40°. In some instances 
another type of track is given, in which one of the 

Fic. 2. 
branches is very short, the other very long. It is 
not unlikely that some of the longest tracks are due 
to hydrogen nuclei. A discussion of the tracks will 
be published very soon in one of the physical journals. 
All the photographs were taken by the Shimizu- 

Fic. 3. 
Wilson method, by means of which many more 
photographs showing views at right angles will soon 
be taken. R. W. RYAn. 
W. D. HARKINS. 
University of Chicago, 
December 23. 

The Age and Area Hypothesis. 
In a paper by the late Prof. D. P. Penhallow, of 
McGill University, Montreal, entitled ‘‘ A Review of 
Canadian Botany from the First Settlement of New 
France to the Nineteenth Century, Part I.” (Pro- 
ceedings and Transactions of the Royal Society of 
Canada for 1887, volume 5, section 4, pp. 45-61, 
1888), the following passage occurs : 
““But Michaux appears to have attached a much 
wider importance to his prospective work, and to 
have regarded it more from a scientific point of view, 
since he had already conceived the idea that the dis- 
tribution of the trees of America should be studied, 
and that it would be possible to ascertain their original 
centres of distribution through careful observation of 
their dimensions and predominance in different parts 
of the country. It was the elaboration of this idea 
that largely led him in so many directions, and over so 
wide a range of territory’ (D. P. Penhallow, Proc. and 
Trans. Roy. Soc. Can., 1887, 5, sect. 4, pp. 55-56, 1888). 
a 
