206 
weismann’s theory of germ-plasm. 
Sep., 1890. 
we lack all the apparatus that make such a variation of service. 
Without strong muscles, rigid vertebrae, copious blood supply, 
and powerful nerves, the additional weight is a serious encum¬ 
brance, a defect that would inevitably cause its possessor to 
become a prey at the very first onslaught of its enemies. To 
suppose that all necessary variations would take place at the 
same time would be to make rather too large a claim on one’s 
credulity, for although the curious phenomena of the correlation 
of variation of organs account for the simultaneous variation 
of related structures, yet variations just suitable in place and 
amount in organs belonging to osseous, muscular, vascular, 
and nervous systems, are difficult to credit. We seem to have 
no alternative, according to Weismann’s theory, except to 
suppose that after a new structure has arisen in one part of 
the body, the organism has to wait an indefinite number of 
generations for the other variations necessary to make it of 
value. The organism would be out of equilibrium in the first 
place, and if further variations occurred in the same direction 
as the first, the balance of functions would be more and more 
disturbed. From the opposite point of view no such difficul¬ 
ties are encountered. If we suppose that after the new struc¬ 
ture has made its appearance, the organism is able, by exercise 
and use, to modify its muscles, &c., to a proper standard of 
efficiency, and transmit the modifications to its offspring, the 
species would readily adjust itself to its new conditions, and 
be ready to receive a fresh instalment of the favourable 
variation if it chanced to occur. 
Weismann explicitly states that if any case can be found 
which cannot be explained by natural selection acting upon 
fortuitous variations, his theory must collapse ; so it is to the 
pursuit of such that I recommend all of those interested in 
biological questions. 
The well-known habit of the Cuckoos of placing their eggs 
in the care of a foster-mother is repeated in the case of the 
Cow-birds # ( Molothrus ), a genus of birds allied to the star¬ 
lings, and widely separated from the cuckoos. Is it probabie 
that the innumerable complex variations upon which such a 
habit must have been built could have been repeated in two 
genera in no way connected with each other ? Had they been 
related, they might have inherited the instinct from a common 
ancestor, but they are not. 
A similar difficult case occurs amongst the Heterosporous 
Cryptogams. Sachs says of these : *• There existed during 
previous geological epochs Horsetails with two kinds of 
Geddes and Thompson, “Evolution of Sex.” 
