接下来，我们要做的是分析划线句子前后语境，并分析划线句子的含义。

　　第一句的上句(背景)：达尔文指出他在简要表达自己观点方面非常吃力。

　　第一句：he believes that this very difficulty may have had the compensating advantage of forcing him to think long and intently about every sentence and thus enabling him to detect errors in reasoning and in his own observations。

　　这句话的主语是he，谓语是believes，宾语是由that引导的复合宾语从句，其中包含由介词宾语引出的to think不定式引导的补语从句和由thus enabling动名词引导的结果从句，可以理解为：

　　他认为或许正因为这种吃力，使他不得不对自己要说的每句话都经过长时间的认真思考，从而能发现自己在具逻辑推理和观察中出现的错误，这反倒成为了他的优点。

　　第一句的下句、第二句的上句(检验上句、承接下句)：他否认自己拥有任何像赫胥黎(Huxley)那样超人的理解能力或智商。

　　第二句：He asserted, also, that his power to follow a long and purely abstract train of thought was very limited, for which reason he felt certain that he never could have succeeded with mathematics。

　　这句话的结构与第一句话相似，主语同样是He，谓语是asserted，宾语是由that引导的复合宾语从句，其中包含由for which reason引导的结果状语从句，和由he felt certain that引导的虚拟语气，可以理解为：