516 EVOLUTION 
evolved. Although Lamarck’s explanation, as based upon the 
use and disuse of organs, applies particularly to animals, he also 
offered an explanation for evolution in plants. In case of plants, 
in which there is no conscious effort as in animals, he assumed 
that changes in the environment affect the body of a plant 
directly and induce modifications which may become sufficiently 
pronounced to characterize a new species. Since species are 
the units of all other groupings of plants or animals, the origin 
of new species results in the origin of new genera, new families, 
new orders, and so on. It is, therefore, obvious that accounting 
for the origin of species accounts for the origin of all those 
differences upon which the various groupings of organisms are 
based. 
Third, Lamarck believed that whatever changes a plant or 
animal made in the form, structure, or function of its body 
were inherited by the offspring. To the succeeding generation 
each generation transmits what it inherited and whatever addi- 
tional modifications it may take on. In this way modifications 
which are only slight at first may become more pronounced in 
succeeding generations, if the conditions remain constant, until 
finally plants or animals so different from their ancestors as to 
form new species may arise. He did not claim that all individ- 
uals taking on new modifications survive but only those pos- 
sessing changes that fit them most perfectly to their environ- 
ment. 
Lamarck’s explanation is unsatisfactory in a number of ways. 
In the first place both observations and experiments furnish 
much evidence that the effects of use and disuse are seldom, if 
at all, inheritable and hence have no permanency such as the 
characters of species have. If the effects of use and disuse are 
not transmitted, the hypothesis that the effects of use and dis- 
use may accumulate from generation to generation also lacks 
support. Also, in case of plants, more recent investigations show 
that modifications that are direct responses to the environment 
are not generally, if at all, inheritable. In the second place his 
explanation does not account for the desire of the animal to 
change its habits, but simply assumes that animals change in 
their desires, and that such changes are also transmitted. 
Darwin’s Explanation. — Although Charles Darwin (Fig. 464) 
was neither the first to believe in evolution nor the first to 
