88 HAECKEL: HIS LIFE, WORK, AND COMPANIONS 
Haeckel’s discussion of the general principles of zoology 
did not, however, divert him from investigating special 
forms of life. The spongida had always attracted his 
attention. For botanists and zoologists during a long time 
doubted whether sponges should rank as members of the 
animal or vegetable kingdom; and, moreover, sponges, 
from their simple structure and plastic form, were fitted to 
throw light on the problem of species. For some time 
prior to Haeckel’s special investigation, sponges had been 
assigned place on the animal border of the protista ; and 
the animal particles of a living sponge were known to be 
‘a subaqueous community of animal life, in which each 
unit takes its stand by a continuous flowing stream drawing 
sustenance from the water as it passes by.’’ For five 
years Haeckel studied the chalk-sponges ; twice taking up 
abode on the sea-coast to have specimens for study in 
their native habitat. His investigations showed how next to 
impossible it is to draw fixed limits for animal species. 
Varieties of the chalk-sponges he found passed from each 
other and back again with such frequency and with such 
differences that he humourously said: ‘‘ You may reckon 
on one genus and three species ; three genera and twenty- 
one species ; or thirty-nine genera with two hundred and 
eighty-one species.’’? His conclusions were that all these 
forms are transitional and were derived from one ancestral 
form, the Olynthus. 
In the prosecution of his zoological work, Haeckel 
became a somewhat notable traveller. In early life his 
sojournings were confined to the shores of the North Sea, 
and to various parts of the Mediterranean coast. But in 
later life he studied the coral reefs of the Red Sea, visited 
the Canary Islands, and made a memorable voyage to 
Ceylon by way of India. His Eastern trip, though but of 
six months duration, furnished matter for a fascinating 
book, as well as supplying him with materials for long 
investigation. His main object was to supplement his 
favourite collections of Mediterranean life with the kindred 
