126 Notices of Boohs. [January, 
of Ly thrum or Primula for the sake of ascertaining whether they 
were specifically distinct, and he found that they could be united 
only with some difficulty, that their offspring were extremely 
sterile, and that the parents and their offspring resembled in a 
whole series of relations crossed species and their hybrid off- 
spring, he might maintain that his varieties had been proved to 
be good and true species, but he would be completely deceived. 
In the second place, as the forms of the same trimorphic or dimor- 
phic hetero-styled species are obviously identical in general 
structure with the exception of the reproductive organs, and as 
they are identical in general constitution (for they live under 
precisely the same conditions), the sterility of their illegitimate 
unions, and that of their illegitimate offspring, must depend ex- 
clusively on the nature of the sexual elements, and on their in- 
compatibility for uniting in a particular manner. And as we 
have just seen that distindt species when crossed resemble in a 
whole series of relations the forms of one and the same species 
when illegitimately united, we are led to conclude that the steri- 
lity of the former must likewise depend exclusively on the in- 
compatible nature of their sexual elements, and not on any 
general difference in constitution or structure. We are, indeed, 
led to this same conclusion by the impossibility of detecting any 
differences sufficient to account for certain species crossing with 
the greatest ease, whilst other closely allied species cannot be 
crossed, or can be crossed only with extreme difficulty. We are 
led to this conclusion still more forcibly by considering the great 
difference which often exists in the facility of crossing recipro- 
cally the same species, for it is manifest in this case that the re- 
sult must depend on the nature of the sexual elements, the male 
element of the one species adding freely on the female element 
of the other, but not so in a reversed direction. And now we see 
that this same conclusion is independently and strongly forti- 
fied by the consideration of the illegitimate unions of trimorphic 
and dimorphic hetero-styled plants. 
It can scarcely be denied that these latest researches of Mr. 
Darwin have dealt a most serious, if not an absolutely fatal, blow 
at the so-called physiological test of species, and consequently 
at the line of absolute demarcation by which some naturalists 
still consider species — as distindt from varieties — to be bounded. 
The Methods of Physical Science. A Ledture delivered at Uni- 
versity College, Bristol, as Introductory to the Course of 
1S77-78. By Sylvanus Thompson, B.Sc., &c. Bristol: 
T. Kerslake and Co. London : Longmans and Co. 
We have here a ledture which brings forward not one novel fadt, 
and probably no conclusion which has not been arrived at and 
