The sun., 1889-12-08


| the one hand. upon the mode Of reproduction
| by fssion which obtains among the mono
plastides (unicellular organismsl and, on the
| other upon the necessity. induced by sexual
| reproduction, for the maintenance OF unicel-
| lular stage In the development OF the polyplas-
tides tmultcellular orRanismsl Finally W.
death itself, and the longer or shorter duration
Of life both depend entirely on adaptation.
Death Is not an essential attribute OF living
matter. 1t is neither necessarily associated
with reproduction. nor necessary conse'
Quence OF It
Or. Weismanns explanation OF the phs-
nomena Of heredity must be gathered from
| five essays published at various times between
| 1883 and 1885, 1t has attracted o great deal OF
| attention. because II distinctly controverts
| view Of biological evolution propounded by
| Lamarck, adopted (althouRh with some hesi-
| tation) by Darwin. and since generally accept
| ed by English and American biologists. Let us
first recall very briefly the prevailing theory.
| and then state the hypothesis which DR.. Weis-
mann would substitute. In Lamarck's opin-
| ion a change in the structure Of any part OF an
| organism was chiefly brought about when
the species in question met with new
conditions OF life. and was thus forced
to assume new habits. Such habits caused
an increased or diminished activity. and.
therefore. a stronger or weaker develop
ment of certain parts, and the modified parts
trere then transmitted to the Offspring. lnas-
much as the offspring continued to live under
the same changed conditions. and kept up the
altered manner Of using the part in question.
the inherited change would be increased In the
| same direction during the course Of their life
and would be further increased in each sue
49, nNaak
change had been effected. In this way. namely
by the assumption Of the transmission Of ac-
quired characters, Lamarck accounted for the
origin Of species.
When Darwin refounded the theory Of or
ganic evolution he believed that he could not
entirely dispense with the Lamarckian princi-
pie OF explanation. though he added the new
and far-reaching principle Of natural selection.
He was disposed to answer in the affirmative
the question whether such changes as those
produced by exercise during an individual life
can be transmitted to the offspring. Hence in
his works use and disuse still play important
parts as direct factors Of transformation. In
addition to natural selection.
The evidence brought forward on behalf Of
the assumption OF the transmission of acquired
characters is examined by DR.. Wetsmanu and
rejected. In one essay he reviews the more
striking supposed instances Of mutilation, and
shows that the alleged evidence Is either ir-
relevant or Inadequate. In another essay he
takes up the pretended botanical proofs OF the
transmission Of acquired characters and dem
onstrates that these also are unacceptable.
His own conception OF the way In which varia-
tion arises, and Is accumulated, Is set forth in-
cidentally in the two papers Just mentioned.
but more elaborately in essays on Heredity."
"On the Continuity OF the Germ Plasm as
~ thA 80
ity," and on "The Significance Of Sexual Pro-
duction in the Theory OF Natural Selection"
His fundamental belief is that heredity de-
pends upon the fact that o small portion Of
the effective portion OF the germ. the germ
plasm. remains unchanged during the develop
ment OF the ovum into an organism. and that
this part OF the germ-plasm serves as founda-
ton from which the germ cells Of the new or
ganism are produced There is. therefore.
continuity OF the germ plasm from one genera
ton to another. One might represent the
germ plasm by the metaphor Of a long, Creep
ing root-stock. from which plants arise at in-
tervals these latter representing the individ-
uas OF successive generations From this
conception Of the conditions OF heredity
it would follow that the transmission
of acquired characters is an impossi-
bility: for. II the germ-plasm Is not
formed anew in each individual but is derived
from that which preceded It its structure. and.
above all. its molecular constitution. cannot |
depend upon the individual In which 1t hap
pens to occur. but such an individual only |
forms as 1t were tho nutritive soil at the ex-
pense OF which the germ plasm grows, while |
the latter possessed its characteristic strue- |
ture from the beginning. viz. before the coma |
mencement OF growth. The tendencies of
heredity of which the germ-plasm is the |
haarar danan1 unon ira pnnlacular at rn otura.
and hence only those characters can be trans
mitted through successive generations which |
have been previously inherited viz. those |
characters which were potentially contained in |
the structure OF the germ pasm. 1t also fol- |
lows. as We have already said. that those other |
characters which have been acquired by the |
influence OF special external conditions during
the lifetime OF the paront cannot be trans
mitted Pt all.
To what, then. does DR.. Weismann attribute
the innate and individual characters which un-
doubtedly occur In animals. and by whose ac-
cumulatlon or intensitication he would himself
account for the origin OF speolesp He finds the
source OF such variations in the form OF re-
production by which the great majority OF ex-
listing organisms are pronagated, viz, in sex-
ual REPRODUCTION. This process consists in
the coalescence OF two distinct Rerm-cells
or perhaps only OF their nuclei These
germ-cels contain the germ substance.
the Germ-plasm. and this again owing to it's
specific molecular structure. is the bearer Of
the hereditary tendencies of the organism,
from which the Rerm-cell has been derived
Thus in sexual reproduction two groups OF
hereditary tendencies are, as 1t were. COM
hined. Dr.Weismann regards this com bination
as the cause OF hereditary individual charac-
ters, and believes that the production OF such
characters Is the true slgnihcRne0 OF sexual
reproduction. Another outcome OF this process