Atheists want evidence for the resultant actions of past activities. Therefore they ask, “Where is the proof that I am suffering and enjoying the resultant actions of past karma?” They have no idea how the subtle body carries the results of the present body’s actions down to the next gross body. The present body may be finished grossly, but the subtle body is not finished; it carries the soul to the next body. Actually, the gross body is dependent on the subtle body. Therefore the next gross body must suffer and enjoy according to the subtle body. The soul is carried by the subtle body continuously until liberated from gross material bondage.
The living entity has two kinds of body–the subtle body and the gross body. Actually, he enjoys through the subtle body, which is composed of mind, intelligence and ego. The gross body is the instrumental outer covering. When the gross body is lost, or when it dies, the root of the gross body–the mind, intelligence, and ego–continues and brings about another gross body. Although the gross bodies apparently change, the real root of the gross body–the subtle body of mind, intelligence, and ego—is always there. The subtle body’s activities–be they pious or impious–create another situation for the living entity to enjoy or suffer in the next gross body. Thus the subtle body continues whereas the gross bodies change one after another.
Since modern scientists and philosophers are too materialistic, and since their knowledge is taken away by the illusory energy, they cannot explain how the gross body is changing. The materialistic philosopher Darwin has tried to study the changes of the gross body, but because he had no knowledge of either the subtle body or the soul, he could not clearly explain how the evolutionary process is going on. One may change the gross body, but he works in the subtle body. People cannot understand the activities of the subtle body, and consequently, they are bewildered as to how the actions of one gross body affect another gross body. The activities of the subtle body are also guided by the Supersoul, as explained in Bhagavadgita (15.15):
sarvasya caham hrdi sannivisto mattah smrtir jnanam apohanam ca
“I am seated in everyone’s heart, and from Me come remembrance, knowledge, and forgetfulness.” Because the Supreme Personality of Godhead as Supersoul is always guiding the individual soul, the individual soul always knows how to act according to the reactions of his past karma. In other words, the Supersoul reminds him to act in such a way. Therefore although there is apparently a change in the gross body, there is a continuation between the lives of an individual soul.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Fourth Canto, Chapter 29 – Text 59 & 60