THE STUDY OF CANCER 5 
define as embodying the essential problem of cancer. So 
distinguished an observer as Campbell de Morgan confessed 
that ‘“‘experiment fails us, and we know nothing of the earliest 
stages of the disease.” The search for a specific cancer cell 
proved futile, according to Wilks, as early as 1868. Investi- 
gation of the minute intra-cellular structures provoked much 
controversy during the last twenty years of the nineteenth 
century, but remained, as it remains to-day, barren of positive 
results. Discussion of the infective or non-infective nature of 
cancer became entirely hypothetical. All attempts to induce 
cancer in animals by inoculation from the human subject 
having led to negative results, the method had been practically 
abandoned. Efforts to elicit experimentally the hypothetical 
(latent) powers of growth of embryonic tissue in support of 
Cohnheim’s hypothesis, or those of groups of adult cells whose 
“organic continuity ” with the tissues was dissolved, as postu- 
lated by Ribbert, showed how limited were the powers of 
growth of‘embryonic and adult tissue. In the case of embryonic 
tissues they showed also that the ultimate cessation of growth 
was accompanied by a differentiation into tissue of adult type. 
The acquisition of new knowledge seemed very remote indeed. 
In spite of the most discouraging outlook, courageous and serious 
resort to experiment had become a pressing necessity at the end 
of the nineteenth century, and, fortunately, the success attending 
subsequent experiments has conferred a positive value on much 
of the apparently negative experimentation described above. 
The study of the phenomena called forth by inoculating the 
tissues of animals into others of the same, and of alien species, 
showed during the closing years of the last century how distinct 
—chemically and biologically—the tissues of one species of 
animal are from those of another, even when nearly allied and 
elaborating their tissues from identical pabulum. The demon- 
stration of the difference appears to have re-directed attention 
to a few earlier experiments on the transmission of tumours 
from one animal to others of the same species. Statements of 
the success attending the deliberate inoculation of human beings 
with portions of their own cancerous tumours were remem- 
bered. Observations made by Hanau, Jenny, Morau were 
remembered as indicating the transference of the disease from 
rats and mice to others of the same species. 
It was by no means clear how those experiments were to be 
